Fixed #12247 -- Corrected the way update queries are processed when the update...

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.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12910 bcc190cf-cafb-0310-a4f2-bffc1f526a37