BUG: String indexing against object dtype may raise AttributeError
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

String indexing may raise `AttributeError`, rather than `KeyError`.

```
df = pd.DataFrame([1], pd.Index([pd.Timestamp('2011-01-01')], dtype=object))
df.index.is_all_dates
# True
df['2011']
# AttributeError: 'Index' object has no attribute '_get_string_slice'
```
