Short circuit issues

This adds a unit test covering #132 (a duplicate of #128):
`ModelMultipleChoiceFilterTests.test_filtering_on_non_required_fields()`

This also adds a unit test covering a related issue:     `ModelMultipleChoiceFilterTests.test_filtering_on_all_of_subset_of_choices()`

This also removes the short circuit logic that causes both of these issues, so the tests now pass.