Update SimpleTestCase.assert method response types

[intgr]

I have made things!

Since PR #909, SimpleTestCase.client.get() etc methods return a class that no longer derives from HttpResponse, but instead HttpResponseBase. The assert* methods taking response should be updated as well.

Usage like this no longer causes an error:

    def test_contains_headers_of_api_response(self):
        resp = self.client.get('/html1')
        self.assertNotContains(resp, '>text/html; charset=utf-8<')

Discovered while trying to update djangorestframework-stubs to use django-stubs git master.