Fixed #7583 -- Corrected the testing docs that referred to the defunct headers...

Fixed #7583 -- Corrected the testing docs that referred to the defunct headers attribute of the response. Added a test case to validate (and document) the new behavior. Thanks to Malcolm for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7900 bcc190cf-cafb-0310-a4f2-bffc1f526a37