BUG: Series/Index contains NaT with object dtype comparison incorrect
- [x] closes #9005 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Series` and `Index` compares `NaT == NaT` as `True` if it has `object` dtype. 

```
pd.Series([pd.NaT])
#0   NaT
# dtype: datetime64[ns]

# OK
pd.Series([pd.NaT]) == pd.NaT
#0    False
# dtype: bool

# NG in object dtype
pd.Series([pd.NaT], dtype=object) == pd.NaT
#0    True
# dtype: bool

# OK
pd.Index([pd.NaT]) == pd.NaT
array([False], dtype=bool)

# NG in object dtype
pd.Index([pd.NaT], dtype=object) == pd.NaT
# array([ True], dtype=bool)
```
