getSupportActivity() should return null when fragment is detached.

[vincentjames501]

The default behavior when a fragment becomes detached from it's parent activity is to set it's managed Activity reference back to null. For example, after a _HoloFragment has been detached, getSupportActivity() returns the original activity where getActivity() returns null. I think the _HoloFragment should override onDetach() and reset it's managed state as Android's Fragment class does normally.