assert_series_equal doesn't distinguish between different NA values
The following, based on tests.indexing.test_datetime.TestDatetimeIndex.test_indexing_with_datetime_tz passes, I think incorrectly

```
now = pd.Timestamp.now()
left = pd.Series([now, pd.NaT, pd.NaT], dtype=object)
right = pd.Series([now, np.nan, np.nan], dtype=object)

tm.assert_series_equal(left, right)
```
