Add support for User models with renamed username field.

cas/backends.py

@@ -234,7 +234,7 @@ def authenticate(self, request, ticket, service):
             return None
 
         try:
-            user = User.objects.get(username__iexact=username)
+            user = User._default_manager.get_by_natural_key(username)
         except User.DoesNotExist:
             # user will have an "unusable" password
             if settings.CAS_AUTO_CREATE_USER:

cas/models.py

@@ -85,13 +85,15 @@ def get_tgt_for(user):
     if not settings.CAS_PROXY_CALLBACK:
         raise CasConfigException("No proxy callback set in settings")
 
+    username = user.get_username()
+
     try:
-        return Tgt.objects.get(username=user.username)
+        return Tgt.objects.get(username=username)
     except ObjectDoesNotExist:
         logger.warning('No ticket found for user {user}'.format(
-            user=user.username
+            user=username
         ))
-        raise CasTicketException("no ticket found for user " + user.username)
+        raise CasTicketException("no ticket found for user " + username)
 
 
 def delete_old_tickets(**kwargs):

cas/tests/test_backend.py

@@ -27,4 +27,4 @@ def test_user_auto_create(self, verify):
 
         with self.settings(CAS_AUTO_CREATE_USER=True):
             user = backend.authenticate('fake', 'fake')
-            self.assertEquals(user.username, username)
+            self.assertEquals(user.get_username(), username)