Fixed #13227 -- Modified ForeignKeys to fully honor the db_prep/prep...

Fixed #13227 -- Modified ForeignKeys to fully honor the db_prep/prep separation introduced by multidb. This was required to ensure that model instances aren't deepcopied as a result of being involved in a filter clause. Thanks to claudep for the report, and Alex Gaynor for the help on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12865 bcc190cf-cafb-0310-a4f2-bffc1f526a37