pandas groupby/aggregation operation changes 'key' column dtype
reference: https://stackoverflow.com/questions/34059704/pandas-groupby-aggregation-operation-changes-key-column-dtype
When I do group and aggregation operation to the original data frame. 
The dtype of the 'key' is changed from int32 to int64. Here is a simple example:

```
df = pd.DataFrame({'id': np.array([3234, 332635, 325993]), 'amount': np.array([34, 43, 32])},
                  index=['a', 'b', 'c'],
                  dtype='int32')
print df.info()

df = df.groupby('id', as_index=False).sum()
print df.info()

```
output is: 
      

>       <class 'pandas.core.frame.DataFrame'>
>       Index: 3 entries, a to c
>       Data columns (total 2 columns):
>       amount    3 non-null int32
>       id        3 non-null int32
>       dtypes: int32(2)
>       memory usage: 48.0+ bytes
>       None
>       
>       <class 'pandas.core.frame.DataFrame'>
>       Int64Index: 3 entries, 0 to 2
>       Data columns (total 2 columns):
>       id        3 non-null int64
>       amount    3 non-null int32
>       dtypes: int32(1), int64(1)
>       memory usage: 60.0 bytes
>       None

pd.show_versions():

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.20.3
pytest: 2.8.5
pip: 9.0.1
setuptools: 20.3
Cython: 0.23.4
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 4.1.2
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.2
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: None
None
