BUG: insert NaT into tz-aware DTI.
this should work (also should test for other index reshaping ops) & for TDI and PI
```
In [7]: pd.DatetimeIndex(['2017-01-01']).insert(0, pd.NaT)
Out[7]: DatetimeIndex(['NaT', '2017-01-01'], dtype='datetime64[ns]', freq=None)

In [8]: pd.DatetimeIndex(['2017-01-01'], tz='UTC').insert(0, pd.NaT)
ValueError: Passed item and index have different timezone
```