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/middleware/views.py

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