Fixed #24931 -- Made namespaced RegexURLResolvers populate callback strings.

Fixed a regression in 2f16ff5a6c.

Thanks Tim Graham for the review.

tests/urlpatterns_reverse/nested_urls.py

@@ -0,0 +1,25 @@
+from django.conf.urls import include, url
+from django.views import View
+
+
+def view1(request):
+    pass
+
+
+def view2(request):
+    pass
+
+
+class View3(View):
+    pass
+
+
+nested = ([
+    url(r'^view1/$', view1, name='view1'),
+    url(r'^view3/$', View3.as_view(), name='view3'),
+], 'backend')
+
+urlpatterns = [
+    url(r'^some/path/', include(nested, namespace='nested')),
+    url(r'^view2/$', view2, name='view2'),
+]