computing mean on non-numeric column turns dataframe into a complex number?
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd                                                                                                                                                              
df = pd.DataFrame({                                                                                                                                                              
    "user":["A", "A", "A", "A", "A"],                                                                                                                                            
    "connections":[18446744.0, 4970.0, 4749.0, 4719.0, 4704.0]                                                                                                                   
})                                                                                                                                                                               
df['connections2'] = df.connections.astype('int64')                                                                                                                              
print("BAD")                                                                                                                                                                     
print(df.mean())                                                                                                                                                                 
                                                                                                                                                                                 
                                                                                                                                                                                 df = pd.DataFrame({                                                                                                                                                              
    "user":["A", "A", "A", "A", "A"],                                                                                                                                            
    "connections":[18446744.0, 4970.0, 4749.0, 4719.0, 4704.0]                                                                                                                   
})                                                                                                                                                                               
df['connections2'] = df.connections                                                                                                                                              
print("GOOD")                                                                                                                                                                    
print(df.mean())                                                                                                                                                                 

## the output is 
BAD                                                                                                                                                                              
user            (9.363467632937e-311+9.3634676604193e-311j)                                                                                                                      
connections                                              0j                                                                                                                      
connections2                                 (3693177.2+0j)                                                                                                                      
dtype: complex128                                                                                                                                                                
GOOD                                                                                                                                                                             
connections     3693177.2                                                                                                                                                        
connections2    3693177.2                                                                                                                                                        
dtype: float64
```
#### Problem description

Having the string,float and int columns in the dataframe  gives me complex numbers on the output of mean.

#### Expected Output
connections     3693177.2                                                                                                                                                        
connections2    3693177.2                                                                                                                                                        
dtype: float64
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
NSTALLED VERSIONS                                                                                                                                                               
------------------                                                                                                                                                               
commit: fa47b8d95e4752d2687b0aee5942dcbb34f61362                                                                                                                                 
python: 3.7.0.final.0                                                                                                                                                            
python-bits: 64                                                                                                                                                                  
OS: Linux                                                                                                                                                                        
OS-release: 4.4.0-1065-aws                                                                                                                                                       
machine: x86_64                                                                                                                                                                  
processor: x86_64                                                                                                                                                                
byteorder: little                                                                                                                                                                
LC_ALL: None                                                                                                                                                                     
LANG: en_US.UTF-8                                                                                                                                                                
LOCALE: en_US.UTF-8                                                                                                                                                              
                                                                                                                                                                                 
pandas: 0.24.0.dev0+515.gfa47b8d                                                                                                                                                 
pytest: None                                                                                                                                                                     
pip: 10.0.1                                                                                                                                                                      
setuptools: 40.0.0                                                                                                                                                               
Cython: 0.28.5                                                                                                                                                                   
numpy: 1.15.0                                                                                                                                                                    
scipy: None                                                                                                                                                                      
pyarrow: None                                                                                                                                                                    
xarray: None                                                                                                                                                                     
IPython: 6.5.0                                                                                                                                                                   
sphinx: None                                                                                                                                                                     
patsy: None                                                                                                                                                                      
dateutil: 2.7.3                                                                                                                                                                  
pytz: 2018.5                                                                                                                                                                     
blosc: None                                                                                                                                                                      
bottleneck: None                                                                                                                                                                 
tables: None                                                                                                                                                                     
numexpr: None                                                                                                                                                                    
feather: None                                                                                                                                                                    
matplotlib: None                                                                                                                                                                 
openpyxl: None                                                                                                                                                                   
xlrd: None                                                                                                                                                                       
xlwt: None                                                                                                                                                                       
xlsxwriter: None                                                                                                                                                                 
lxml: None                                                                                                                                                                       
bs4: None                                                                                                                                                                        
html5lib: 1.0.1                                                                                                                                                                  
sqlalchemy: None                                                                                                                                                                 
pymysql: None                                                                                                                                                                    
psycopg2: None                                                                                                                                                                   
jinja2: 2.10                                                                                                                                                                     
s3fs: None                                                                                                                                                                       
fastparquet: None                                                                                                                                                                
pandas_gbq: None                                                                                                                                                                 
pandas_datareader: None                                                                                                                                                          
gcsfs: None        
</details>
