round function on a dtype: object does not return the expected result without further information
Sometimes pandas ``round`` function does not return the expected results without informing me about the reason.  Consider the following example:  
```python
import pandas as pd
import numpy as np 

output = pd.DataFrame(index = ['foo'], columns = ['bar'])
output.loc['foo', 'bar'] = 1.2345
output.round(2) 
Out[1]: 
        bar
foo  1.2345
```
I think the reason for the unexpected result is the type of  ``output`` (or it's elements?), which is: 
```
In [2]: output.loc[:,'bar']
Out[2]: 
foo    1.2345
Name: bar, dtype: object
```
To obtain the expected result, I write: 
```
output = output.astype(np.double)
output.round(2) 
Out[3]: 
      bar
foo  1.23
```
Can somebody maybe say whether the first result is indeed intended?  I wonder if ``pandas.round`` can warn the user that it does not operate on a ``dtype: object``? 

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-128-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.13.1
scipy: 0.19.0
statsmodels: 0.8.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.2
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
boto: 2.46.1
pandas_datareader: 0.2.1

</details>
