Fixed #13615 -- Clarified test assertion text to avoid confusion when response...

Fixed #13615 -- Clarified test assertion text to avoid confusion when response content isn't a web page. Thanks to DaNmarner for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13512 bcc190cf-cafb-0310-a4f2-bffc1f526a37