Make SuccessMessageMixin fully compatible with FormMixin (#86)

This ensures that the order in which these mixins are included
into a derrived class does not matter and ends up more accurately
reflecting the return type of SuccessMessageMixin in the process
(its code doesn't appear to enforce that the returned response
is a redirect).

This provides a fix to a secondary aspect of
https://github.com/mkurnikov/django-stubs/issues/79.

test-data/typecheck/messages.test

@@ -0,0 +1,10 @@
+[CASE successmessagemixin_compatible_with_formmixin]
+from django.views.generic.edit import FormMixin
+from django.contrib.messages.views import SuccessMessageMixin
+
+class FormFirstView(FormMixin, SuccessMessageMixin):
+    pass
+
+class SuccessMessageFirstView(FormMixin, SuccessMessageMixin):
+    pass
+[/CASE]