che's mod_proxy can do this), when the address
        the HTTP client is using is different than the one we're listening on.

        For example, Apache may be listening on https://www.example.com/, and
        then forwarding requests to http://localhost:8080/, but we don't want
        HTML produced by Twisted to say b'http://localhost:8080/', they should
        say b'https://www.example.com/', so we do::

           request.setHost(b'www.example.com', 443, ssl=1)

        @type host: C{bytes}
        @param host: The value to which to change the host header.

        @type ssl: C{bool}
        @param ssl: A flag which, if C{True}, indicates that the request is
            considered secure (if C{True}, L{isSecure} will return C{True}).
        iť