Fixed #21157 -- Fixed problems with ResolverMatch

- Fixed bug in get_callable() that caused resolve() to put a string
  in ResolverMatch.func.
- Made ResolverMatch.url_name match the actual url name (or None).
- Updated tests that used the string value in ResolverMatch.func, and
  added regression tests for this bug.
- Corrected test urls whose dummy view paths caused failures (behavior
  that was previously masked by this bug).

tests/model_permalink/urls.py

@@ -1,5 +1,5 @@
 from django.conf.urls import url
 
 urlpatterns = [
-    url(r'^guitarists/(\w{1,50})/$', 'unimplemented_view_placeholder', name='guitarist_detail'),
+    url(r'^guitarists/(\w{1,50})/$', 'model_permalink.views.empty_view', name='guitarist_detail'),
 ]

tests/model_permalink/views.py

@@ -0,0 +1,5 @@
+from django.http import HttpResponse
+
+
+def empty_view(request, *args, **kwargs):
+    return HttpResponse('')