Fixed #17838 - prefetch_related fails for GenericForeignKeys when related object id is not a CharField/TextField

Thanks to mkai for the report and debugging, and tmitchell and Przemek
Lewandowski for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17744 bcc190cf-cafb-0310-a4f2-bffc1f526a37