BUG: Index may ignore specified datetime/timedelta dtypes
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Index` ignores specified `dtype` if it is `datetime64` (normal and tz) or `timedelta64`. This PR makes it consistent with `DatetimeIndex` and `TimedeltaIndex`.

```
pd.Index([1, 2, 3], dtype='datetime64[ns, US/Eastern]')
# Index([1, 2, 3], dtype='object')
pd.Index([1, 2, 3], dtype='datetime64[ns]')
# Index([1, 2, 3], dtype='object')
pd.Index([1, 2, 3], dtype='timedelta64[ns]')
# Int64Index([1, 2, 3], dtype='int64')
```

Also, fixed `MultiIndex.get_level_values` not to pass unnecessary `tz` and `freq`.
