Index._maybe_cast_slice_bound switched meaning of left/right
'left' and 'right' changed meaning in `DatetimeIndex._maybe_cast_slice_bound`.  Was this intentional?
### Pandas 0.18

``` python
In [1]: import pandas as pd

In [2]: index = pd.DatetimeIndex(freq='1H', periods=0, end='2015')

In [3]: index._maybe_cast_slice_bound('2015-01-02', 'right', 'loc')
Out[3]: Timestamp('2015-01-02 23:59:59.999999999')

In [4]: index._maybe_cast_slice_bound('2015-01-02', 'left', 'loc')
Out[4]: Timestamp('2015-01-02 00:00:00')
```
### Pandas 0.19

``` python
In [1]: import pandas as pd

In [2]: index = pd.DatetimeIndex(freq='1H', periods=0, end='2015')

In [3]: index._maybe_cast_slice_bound('2015-01-02', 'right', 'loc')
Out[3]: Timestamp('2015-01-02 00:00:00')

In [4]: index._maybe_cast_slice_bound('2015-01-02', 'left', 'loc')
Out[4]: Timestamp('2015-01-02 23:59:59.999999999')
```
