[1.0.X] Fixed #9607 -- Added documentation for the ``extra`` argument in test...

    Once you have a ``Client`` instance, you can call any of the following

    methods:

    .. method:: Client.get(path, data={})

    .. method:: Client.get(path, data={}, **extra)

        Makes a GET request on the provided ``path`` and returns a ``Response``

        object, which is documented below.

            /customers/details/?name=fred&age=7

    .. method:: Client.post(path, data={}, content_type=MULTIPART_CONTENT)

        The ``extra`` keyword arguments parameter can be used to specify

        headers to be sent in the request. For example::

            >>> c = Client()

            >>> c.get('/customers/details/', {'name': 'fred', 'age': 7},

            ...       HTTP_X_REQUESTED_WITH='XMLHttpRequest')

        ...will send the HTTP header ``HTTP_X_REQUESTED_WITH`` to the

        details	view, which is a good way to test code paths that use the

        :meth:`django.http.HttpRequest.is_ajax()` method.

    .. method:: Client.post(path, data={}, content_type=MULTIPART_CONTENT, **extra)

        Makes a POST request on the provided ``path`` and returns a

        ``Response`` object, which is documented below.

        Note that you should manually close the file after it has been provided

        to ``post()``.

        The ``extra`` argument acts the same as for :meth:`Client.get`.

    .. method:: Client.login(**credentials)

        .. versionadded:: 1.0