Fixed #5937 -- When filtering on generic relations, restrict the target objects to those with the right content type.

This isn't a complete solution to this class of problem, but it will do for
1.0, which only has generic relations as a multicolumn type. A more general
multicolumn solution will be available after that release.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8608 bcc190cf-cafb-0310-a4f2-bffc1f526a37