[1.1.X] Fixed #12247 -- Corrected the way update queries are processed when...

[1.1.X] Fixed #12247 -- Corrected the way update queries are processed when the update only refers to attributes on a base class. Thanks to jsmullyan for the report, and matiasb for the fix.

Backport of r12910 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12911 bcc190cf-cafb-0310-a4f2-bffc1f526a37