[1.2.X] Fixed #14471 -- Corrected a regression in the use of methods on custom...

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

Backport of r14389 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14390 bcc190cf-cafb-0310-a4f2-bffc1f526a37