Series[datetime64[ns, tz]].__setitem__ fails to coerce for ndarray value
What's the expected output for

```python
In [9]: a = pd.Series(pd.DatetimeIndex(['2000', '2000'], tz="UTC"))

In [10]: a.loc[:] = np.array([pd.Timestamp('2000', tz="UTC")] * 2)
```

On master, we coerce `self` to `object` and then set.

```python
In [11]: a
Out[11]:
0    2000-01-01 00:00:00+00:00
1    2000-01-01 00:00:00+00:00
dtype: object
```

Should we instead coerce `value`, rather than self?
