.loc slice with datetime string with DataFrame of length 1 gives an empty DataFrame
I've already raised the problem at https://github.com/pandas-dev/pandas/issues/13929#issuecomment-295326414 but it turned out to be a different issue.

#### The following test code illustrates the problem

```python
import pandas as pd
from pandas.util import testing as tm

from datetime import datetime
df = pd.DataFrame(columns=['1'],
                  index=pd.date_range('2016-10-01T00:00:00',
                                      '2016-10-01T23:59:59'))
df_sliced_dt = df.loc[datetime(2016,10,1):]
tm.assert_frame_equal(df, df_sliced_dt)

df = pd.DataFrame(columns=['1'],
                  index=pd.date_range('2016-10-01T00:00:00',
                                      '2016-10-01T23:59:59'))
df_sliced_str = df.loc['2016-10-01T00:00:00':]
tm.assert_frame_equal(df, df_sliced_str)
```


