Fixed #17972 -- Ensured that admin filters on a foreign key respect the...

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.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17854 bcc190cf-cafb-0310-a4f2-bffc1f526a37