Middleware regression test fixes:

 * Added a models.py file so the tests run.
 * Reset `settings.DEBUG` to `False` at the end of the `test_append_slash_no_redirect_on_POST_in_DEBUG` test so it doesn't bleed over to other tests.
 * Removed unused import.
 * Minor docstring fixes.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6991 bcc190cf-cafb-0310-a4f2-bffc1f526a37