Fixed #12209 -- Made the through attribute on a m2m relation into a property...

Fixed #12209 -- Made the through attribute on a m2m relation into a property to ensure that the fully resolved through model is always provdided. Thanks to dgouldin for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11736 bcc190cf-cafb-0310-a4f2-bffc1f526a37