added tests for respond_with

Noticed locally that active_model_serializers :serializer and :each_serializer options don't appear to be working with respond_with as might be expected, so added a few tests for respond_with that (1) uses the default serializer, (2) uses a custom each_serializer, and (3) uses a custom serializer.

Wish I could say that I had the fix to make these tests work, but if someone could provide some guidance, I'd be glad to try to add support as soon as I have time. Thanks!