Fix support for proxy models in formsets and admin inlines

(cherry picked from commit c2768f8101)

polymorphic/admin/generic.py

@@ -42,7 +42,7 @@ class GenericPolymorphicInlineModelAdmin(PolymorphicInlineModelAdmin, GenericInl
             Expose the ContentType that the child relates to.
             This can be used for the ``polymorphic_ctype`` field.
             """
-            return ContentType.objects.get_for_model(self.model)
+            return ContentType.objects.get_for_model(self.model, for_concrete_model=False)
 
         def get_formset_child(self, request, obj=None, **kwargs):
             # Similar to GenericInlineModelAdmin.get_formset(),

polymorphic/admin/helpers.py

@@ -42,7 +42,7 @@ class PolymorphicInlineAdminFormSet(InlineAdminFormSet):
         """
         for form, original in zip(self.formset.initial_forms, self.formset.get_queryset()):
             # Output the form
-            model = original.get_real_concrete_instance_class()
+            model = original.get_real_instance_class()
             child_inline = self.opts.get_child_inline_instance(model)
             view_on_site_url = self.opts.get_view_on_site_url(original)