CLN: simplify MultiIndex._shallow_copy
``MultiIndex._shallow_copy``  doesnt pass on kwargs, unless ``values`` is passed also, but just silently drops the kwargs. This makes ``_shallow_copy`` currently a bit unreliable.

This PR fixes that problem by calling ``.copy`` directly and passing kwargs to ``copy``and not passing the call through ``.view``. Also, ``_shallow_copy`` doesn't call ``.view`` on other index types, so this PR makes ``MultiIndex._shallow_copy`` conform with other index types.