DatetimeIndex doesn't validate `dtype` argument
```python
In [2]: pd.DatetimeIndex([1, 2], dtype=object())
Out[2]: DatetimeIndex(['1970-01-01 00:00:00.000000001', '1970-01-01 00:00:00.000000002'], dtype='datetime64[ns]', freq=None)
```

this should probably raise.