GroupRequiredMixin bug with sequences in group_required

Hi,

I've fixed a bug in the GroupRequiredMixin by which the check_membership method failed to validate a user if group_required was a sequence (list or tuple) of groups. The implementation validates ANY user whose group(s) appear in group_required, as specified in https://github.com/brack3t/django-braces/issues/35.

This bug went undetected because of errors in the test suite. I've also fixed and explained them in the commits.

If you feel like you need anything else to accept the pull request I am open to refactorings.