[1.2.X] Fixed #15161 - Corrected handling of ManyToManyField with through...

[1.2.X] Fixed #15161 - Corrected handling of ManyToManyField with through table using to_field on its ForeignKeys. Thanks to adehnert for the report.

Backport of r15330 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15331 bcc190cf-cafb-0310-a4f2-bffc1f526a37