[1.2.X] Fixed #15103 - SuspiciousOperation with limit_choices_to and raw_id_fields

Thanks to natrius for the report.

This patch also fixes some unicode bugs in affected code.

Backport of [15347] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15349 bcc190cf-cafb-0310-a4f2-bffc1f526a37