Fixed filter_horizontal and filter_vertical handling in admin so that they can

be lists (aren't required to be tuples). It's less error-prone to use one item
lists than one-tuples, but this caused a crash.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8011 bcc190cf-cafb-0310-a4f2-bffc1f526a37