Pandas replace with string and integers - incorrect behavior?
I encountered a potentially incorrect behavior of pandas replace with strings and integers. If the dataframe has both 0 (integer) and '0' (strings) then replace '0' affects both strings and integers. Here's how it goes:

`In [1]: df = pd.DataFrame({'numbers' : [0, 1, 2, 0], 'strings' : ['0', 1, 2, '0']})`

To check that it's indeed the correct setup:

```
In [2]: df.dtypes
Out [2]:
numbers     int64
strings    object
dtype: object

```
And check individual values:
```
In [3]: type(df['numbers'][0])
Out[3]: numpy.int64
In [4]: type(df['strings'][0])
Out[4]: str
```
Now, do replace:
```
In [5]: df.replace(to_replace='0', value=np.NaN, inplace=True)
In [6]: df.head()
Out[6]: 
   numbers  strings
0      NaN      NaN
1        1        1
2        2        2
3      NaN      NaN
```

As you can see, it replaced both strings and integers, however should have worked only on the strings. If we try same on integers, it works correctly:

```
In [7]: df = pd.DataFrame({'numbers' : [0, 1, 2, 0], 'strings' : ['0', 1, 2, '0']})
...: df.replace(to_replace=0, value=np.NaN, inplace=True)
...: print df.head()
Out [7]:   
numbers strings
0      NaN       0
1        1       1
2        2       2
3      NaN       0
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.0
nose: None
pip: 8.1.2
setuptools: 3.6
Cython: None
numpy: 1.11.2
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 4.0.3
sphinx: None
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: 1.0.0
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: 1.0b8
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None

</details>
