Fixed #7378 -- Use the "to_field" where appropriate on reverse relations.

Patch from mturtle@gmail.com. The remaining uses of "%s__pk" in
fields/related.py all look safe, since they are for many-to-many fields, which
doesn't take "to_field" as a parameter.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7785 bcc190cf-cafb-0310-a4f2-bffc1f526a37