BUG: DatetimeIndex cast to object dtype raises/wrong for tzaware

```
dti = pd.date_range('2016-01-01', periods=3, tz='US/Central')

>>> pd.Index(dti, dtype=object)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/pandas/core/indexes/base.py", line 294, in __new__
    dtype=dtype, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/indexes/datetimes.py", line 453, in __new__
    raise ValueError("cannot localize from non-UTC data")
ValueError: cannot localize from non-UTC data

>>> np.array(dti, dtype=object)
array([1451628000000000000L, 1451714400000000000L, 1451800800000000000L],
      dtype=object)
```

I expected these to match `pd.Index(list(dti), dtype=object)` and `np.array(list(dti))`, respectively.
