The old process_request method for middleware was deprecated with the

move to MIDDLEWARE from the old MIDDLEWARE_CLASSES. Our testcase

create_http_request method does a bunch of stuff to build a fake HTTP

request, including running the session and message middleware to

populate attributes on the request. This change moves that invocation

over to the new style (instantiate with a get_response method, and

then treat as a callable).

Ran unit tests.