Make ManyToManyField model references more robust.

In r16679 a test was added to verify something had been fixed when a
unicode string type was passed in as a model name. The name has to be
ASCII convertible, but in Python 2.6 and earlier, it must also have str
type.

This commit fixes the problem for earlier Python versions and is almost
identical to a patch from b.leskes in #6045.

Fixes #16689. Refs #6045.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16682 bcc190cf-cafb-0310-a4f2-bffc1f526a37