[1.8.x] Fixed a typo in the test responses docs.

The ``get()`` and ``post()`` methods both return a ``Response`` object. This

``Response`` object is *not* the same as the ``HttpResponse`` object returned

Django views; the test response object has some additional data useful for

by Django views; the test response object has some additional data useful for

test code to verify.

Specifically, a ``Response`` object has the following attributes: