[1.2.X] Fixed #14948 -- Fixed a couple of cases where invalid model classes...

[1.2.X] Fixed #14948 -- Fixed a couple of cases where invalid model classes were passed to the database router when collecting reverse foreign key and many to many relationships. Thanks shell_dweller for the report and Harm Geerts for the patch.

This also enhances tests added in r15186.

Code in SVN trunk doesn't suffer from this problem because it was refactored in r14507.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15207 bcc190cf-cafb-0310-a4f2-bffc1f526a37