Mean of dataframe with object column returns dataframe of complex128 dtype
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'name': ['a']})
df['value'] = 0.5
print(df.mean(numeric_only=True))
print()
print(df.mean())
```
```
value    0.5
dtype: float64

name     2.320359906103466e+77j
value                  (0.5+0j)
dtype: complex128
```
#### Problem description

Computing the mean of a dataframe that has an object dtype column converts the whole dataframe to a complex type. This is unexpected and different from previous behavior (used to just drop the object columns)

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Darwin
OS-release: 17.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+608.g1a12c41d2
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.4
numpy: 1.15.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.7.5
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.6
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.4
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: 1.2.9
pymysql: None
psycopg2: 2.7.5 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
