Fixed #14471 -- Corrected a regression in the use of methods on custom...

Fixed #14471 -- Corrected a regression in the use of methods on custom managers on related querysets. Thanks to Lucky for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14389 bcc190cf-cafb-0310-a4f2-bffc1f526a37