BUG: CategoricalIndex.get_loc returns array even if it is unique
- [x] related to #11558
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Made `CategoricalIndex.get_loc` to return the same result as normal `Index`.

```
data2 = pd.DataFrame([[1,2,3],[3,4,5]], index=pd.Categorical(['one', 'two']))

# in current master, it returns DataFrame with 1 row 
data2.ix['one']
#      0  1  2
# one  1  2  3

# after this PR, it returns Series
data2.ix['one']
#0    1
#1    2
#2    3
# Name: one, dtype: int64
```

@sinhrks this looks good. but is this the correct reference issue: #11588 ?
 Ah, related to #11558, but can't close only by this PR. Fixed the doc and comment.
 @sinhrks thanks! nice PR as usual
