Fixed #20777 -- Admin proxy model deletion regression

Added proxy_models tests by Harm Geerts <github@geertswei.nl>.

tests/proxy_models/urls.py

@@ -0,0 +1,7 @@
+from django.conf.urls import patterns, include
+
+from django.contrib import admin
+
+urlpatterns = patterns('',
+    (r'^admin/', include(admin.site.urls)),
+)