KeyError when setting values with loc
#### Code Sample, a copy-pastable example if possible

```python
# following works as expected:
import pandas as pd
index_oct = pd.date_range('2016-10-01', '2016-11-01', freq='H')
frame_oct = pd.DataFrame(index=index_oct, columns=pd.Index([], name='Id'))
frame_oct.loc[:, '111916'] = pd.Series(2., index=index_oct)

# but doing the same in November returns a KeyError:
index_nov = pd.date_range('2016-11-01', '2016-12-01', freq='H')
frame_nov = pd.DataFrame(index=index_nov, columns=pd.Index([], name='Id'))
frame_nov.loc[:, '111916'] = pd.Series(2., index=index_nov)

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\IPython\core\interactiveshell.py", line 2881, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-4-80822f072d9c>", line 3, in <module>
    frame_nov.loc[:, '111916'] = pd.Series(2., index=index_nov)
  File "C:\Python27\lib\site-packages\pandas\core\indexing.py", line 141, in __setitem__
    self._setitem_with_indexer(indexer, value)
  File "C:\Python27\lib\site-packages\pandas\core\indexing.py", line 300, in _setitem_with_indexer
    indexer, self.obj.axes)
  File "C:\Python27\lib\site-packages\pandas\core\indexing.py", line 1854, in convert_from_missing_indexer_tuple
    return tuple([get_indexer(_i, _idx) for _i, _idx in enumerate(indexer)])
  File "C:\Python27\lib\site-packages\pandas\core\indexing.py", line 1851, in get_indexer
    return (axes[_i].get_loc(_idx['key']) if isinstance(_idx, dict) else
  File "C:\Python27\lib\site-packages\pandas\indexes\base.py", line 2136, in get_loc
    return self._engine.get_loc(self._maybe_cast_indexer(key))
  File "pandas\index.pyx", line 132, in pandas.index.IndexEngine.get_loc (pandas\index.c:4433)
  File "pandas\index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas\index.c:4279)
  File "pandas\src\hashtable_class_helper.pxi", line 732, in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:13742)
  File "pandas\src\hashtable_class_helper.pxi", line 740, in pandas.hashtable.PyObjectHashTable.get_item (pandas\hashtable.c:13696)
KeyError: '111916'

```
#### Problem description

Apparently '111916' is interpreted as 19 November 2016 in case this date is contained in the index. Nevertheless, I would not expect this to fail as a column is referred to and not the index

#### Expected Output

No KeyError, but the same behaviour as for frame_oct

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

<details>
pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.8.0
xarray: None
IPython: 5.2.2
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: None
numexpr: None
matplotlib: 2.0.0
openpyxl: 2.4.2
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.6.4
bs4: 4.5.3
html5lib: 0.999999999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None

</details>
