Test that proxy models derived from PolyMorphicBase classes

Test that proxy models derived from PolyMorphicBase classes
can call get_real_instance() and get_real_instance_class()

I wasn't quite sure where to add this test.  It looked like ```test_get_real_instances``` and ```test_proxy_models``` were pretty specific so I created a new test ```test_proxy_get_real_instance_class```.

If this isn't ideal let me know and I can move it to a better location.