an merge problem, about "read_csv" and "astype"
-  I have two csv file below:   

test1.csv
```
COL1,COL2
10124420001034504343,1
10124420001095178632,2
10124420000004627498,3
```
test2.csv
```
COL1,COL2
10124420001034504343,a
10124420001095178632,b
20124400001011907368,c
```

- why df1 and df3 are different? (merge(d1, d2) can't get the output I want)
```
In [1]: import pandas as pd

In [2]: df1 = pd.read_csv('./test1.csv')

In [3]: df2 = pd.read_csv('./test2.csv')

In [4]: df1.dtypes
Out[4]:
COL1    uint64
COL2     int64
dtype: object

In [5]: df2.dtypes
Out[5]:
COL1    object
COL2    object
dtype: object

In [6]: df1['COL1'] = df1['COL1'].astype('object')

In [7]: df1.dtypes
Out[7]:
COL1    object
COL2     int64
dtype: object

In [8]: merge1 = pd.merge(df1, df2, on='COL1')

In [9]: merge1.info()
<class 'pandas.core.frame.DataFrame'>
Index: 0 entries
Data columns (total 3 columns):
COL1      0 non-null object
COL2_x    0 non-null int64
COL2_y    0 non-null object
dtypes: int64(1), object(2)
memory usage: 0.0+ bytes
```

```
In [10]: df3 = pd.read_csv('./test1.csv', dtype={'COL1': object})

In [11]: df3.dtypes
Out[11]:
COL1    object
COL2     int64
dtype: object

In [12]: merge2 = pd.merge(df3, df2, on='COL1')

In [13]: merge2.info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 2 entries, 0 to 1
Data columns (total 3 columns):
COL1      2 non-null object
COL2_x    2 non-null int64
COL2_y    2 non-null object
dtypes: int64(1), object(2)
memory usage: 64.0+ bytes
```

#### Output of ``pd.show_versions``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Windows
OS-release: 8.1
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.21.0
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.13.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
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
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
