[1.7.x] Fixed #13755 -- Added a tip for caching responses that vary on AJAX.

   If you write your own XMLHttpRequest call (on the browser side), you'll

   have to set this header manually if you want ``is_ajax()`` to work.

    If a response varies on whether or not it's requested via AJAX and you are

    using some form of caching like Django's :mod:`cache middleware

    <django.middleware.cache>`, you should decorate the view with

    :func:`vary_on_headers('HTTP_X_REQUESTED_WITH')

    <django.views.decorators.vary.vary_on_headers>` so that the responses are

    properly cached.

.. method:: HttpRequest.read(size=None)

.. method:: HttpRequest.readline()

.. method:: HttpRequest.readlines()