ADDED: Improved support for reading and writing HTTP status codes.

    Reading
    -------

    All codes that should be read according to RFC 7231 can now be read.
    This used to be only the most commonly used ones.
    Codes that are not explicitly defined are reduced to
    their x00 class code (as per RFC 7231).

    Status codes that are read are now also returned in http_get/3 and
    http_post/4 by option `status_code` (consistent with http_open/3).

    Writing
    -------

    All legal codes (RFC 7231) can now be written.
    In addition, all codes within the 2xx range are interpreted
    as conveying a reply that is 'ok'.

    Examples
    --------

        some_http_handler(Request):-
          reply_json(json{}, [status(555)]).

        ?- http_get(SomeUrl, _Reply, [status_code(X)]).
        X = 555.