Fixed #15826 -- Made `assertContains` and `assertNotContains` work with `SimpleTemplateResponse` by forcing it to be rendered if it hasn't been rendered yet. Thanks to bmihelac for the report, to mmcnickle for the initial patch and tests, and to Carl Meyer for the guidance.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17025 bcc190cf-cafb-0310-a4f2-bffc1f526a37