Fixed #16128 - Correctly cascade-delete proxy models as if they were the...

Fixed #16128 - Correctly cascade-delete proxy models as if they were the concrete model class. Thanks xkennyx for the report, and Aymeric Augustin, Claude Paroz, Adam Nelson, jaap3, and Anssi Kääriäinen for work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17664 bcc190cf-cafb-0310-a4f2-bffc1f526a37