[1.3.X] Fixed #17972 -- Ensured that admin filters on a foreign key respect...

[1.3.X] Fixed #17972 -- Ensured that admin filters on a foreign key respect the to_field attribute. This fixes a regression introduced in [14674] and Django 1.3. Thanks to graveyboat and Karen Tracey for the report.

Backport of r17854 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@17857 bcc190cf-cafb-0310-a4f2-bffc1f526a37