Fixed #7156 -- Normalise file paths before returning them in models.

This avoids problems with a database that was created on Unix being used on
Windows and giving interesting results. Patch from fcaprioli@alice.it.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7795 bcc190cf-cafb-0310-a4f2-bffc1f526a37