Added an optional `backend` argument to login().

Thanks to Tim Graham for the review.

urlparse() fails with an AttributeError ("'__proxy__' object has no
attribute 'decode'") if reverse_lazy is used to look up the URL
(this is exactly the same problem that caused ticket #18776). The
solution is to use force_str() on the path before handing it to
urlparse().

Thanks Carl Meyer for the report and Tim Graham for the review.

Thanks KevinEtienne for the suggestion.

This is a reliqua from the early days of the modeltests/regressiontests era.

Thanks mrmachine for the suggestion.

Thanks Carl Meyer for the suggestion and Alex Gaynor and Carl for reviews.

This (nearly) completes the work to isolate all the test modules from
each other. This is now more important as importing models from another
module will case PendingDeprecationWarnings if those modules are not in
INSTALLED_APPS. The only remaining obvious dependencies are:

- d.c.auth depends on d.c.admin (because of the is_admin flag to some
  views), but this is not so important and d.c.admin is in
  always_installed_apps
- test_client_regress depends on test_client. Eventually these should
  become a single module, as the split serves no useful purpose.

Originating WSGIRequests are now attached to the ``wsgi_request`` attribute of
the ``HttpResponse`` returned by the testing client.

Thanks rvdrijst for the suggestion.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

Thanks Gunnar Scherf for the report and the suggested patch.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

All request methods of ``django.test.client.Client`` receive a ``secure``
argument that defaults to ``False`` indicating whether or not to make the
request through https.
Thanks Aymeric Augustin for the review.

* Removed trailing whitespace.
* Added newline to EOF if missing.
* Removed blank lines at EOF.
* Removed some stray tabs.