BUG: scalar assignment of a tz-aware is object dtype
[3] should be a ``datetime64[ns, UTC]``

```
In [1]: df = pd.DataFrame({'A': [0, 1]})

In [3]: df['now'] = pd.Timestamp('20130101', tz='UTC')

In [4]: df
Out[4]: 
   A                        now
0  0  2013-01-01 00:00:00+00:00
1  1  2013-01-01 00:00:00+00:00

In [5]: df.dtypes
Out[5]: 
A       int64
now    object
dtype: object

In [6]: df['now2'] = pd.DatetimeIndex([pd.Timestamp('20130101', tz='UTC')]).repeat(len(df))

In [7]: df.dtypes
Out[7]: 
A                     int64
now                  object
now2    datetime64[ns, UTC]
dtype: object
```

