Wired KeyError when indexing DatetimeIndex
```python
import pandas as pd
from datetime import *

#Creat a sample DataFrame, df.
d={'foo':1, 'bar':2}
index=['2017-09-13']
df=pd.DataFrame(data=d,index=index)
df.index=pd.DatetimeIndex(df.index)

#Try indexing it with year-month, works
df['2017-09']

'''
            bar  foo
2017-09-13    2    1
'''

#Try indexing it with year-month-day, reprots a KeyError
 df['2017-09-13']
 
 '''
 
  File "d:\Users\...\Anaconda2\lib\site-packages\pandas\core\indexes\base.py", line 2444, in get_loc
    return self._engine.get_loc(self._maybe_cast_indexer(key))

  File "pandas\_libs\index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5280)

  File "pandas\_libs\index.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5126)

  File "pandas\_libs\hashtable_class_helper.pxi", line 1210, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20523)

  File "pandas\_libs\hashtable_class_helper.pxi", line 1218, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20477)

    KeyError: '2017-09-13'
 '''

```
#### Problem description
Using python 2.7, spyder, all packages installed from Anaconda. 
When using a string specifying the date to index a DataFrame with DatetimeIndex , it returns KeyError.

Expected to return the data of that specific date.


<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.20.3
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.26
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 5.3.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: 0.5.0
</details>
