[1.7.x] Fixed #20609 -- Added instructions for using AnonymousUser with RequestFactory.

Backport of a39df37049 from master

docs/topics/testing/advanced.txt

@@ -37,7 +37,7 @@ Example
 
 The following is a simple unit test using the request factory::
 
-    from django.contrib.auth.models import User
+    from django.contrib.auth.models import AnonymousUser, User
     from django.test import TestCase, RequestFactory
 
     class SimpleTest(TestCase):
@@ -55,6 +55,10 @@ The following is a simple unit test using the request factory::
             # logged-in user by setting request.user manually.
             request.user = self.user
 
+            # Or you can simulate an anonymous user by setting request.user to
+            # an AnonymousUser instance.
+            request.user = AnonymousUser()
+
             # Test my_view() as if it were deployed at /customer/details
             response = my_view(request)
             self.assertEqual(response.status_code, 200)