Fixed Django 1.5 test failures and bug in nested resource tests

* I started with the commits in toastdriven/django-tastypie/pull/681, which by themselves result in failed tests (toastdriven/django-tastypie/tests/validation/tests.py - see test output in a [gist](https://gist.github.com/jlafon/5225293)).
* The tests failed because of a bug fix in Django ([see ticket](https://code.djangoproject.com/ticket/18153)) involving related resources.  Specifically, this commit https://github.com/django/django/commit/3190abcd75b1fcd660353da4001885ef82cbc596 was the cause.
* This commit (https://github.com/jlafon/django-tastypie/commit/431aaa9a17498a475df3ab26529cf0a94175a7af) fixes the failed tests and avoids relying on the bug in Django 1.5  
* Tested with Django 1.4.2 and Django 1.5
* I realize this may not be the ideal solution.