Fixed #16299 -- Ensure that unicode strings can be used to identify classes in...

Fixed #16299 -- Ensure that unicode strings can be used to identify classes in ForeignKey and ManyToManyFields. Unicode strings aren't actually legal as class names, but this is an issue if you use from __future__ import unicode_literals in your models.py file. Thanks to Martijn Bastiaan for the report, and Anthony Briggs for the final patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16663 bcc190cf-cafb-0310-a4f2-bffc1f526a37