Add support for Preserving the client provided Host header.

Add support for Preserving the client provided Host header.

* modules/proxy/mod_serf.c
    (serf_config_t): Add preservehost member.
    (setup_request): If preservehost is set, use the client provided Host header,
        otherwise use the one from the configuration.
    (is_true): New helper function for decoding true/false strings.
    (add_pass): Change to an argv configuration function, check for not enough
        args, and parse everything after the URI into key/value pairs.
    (create_dir_config): Default to setting preservehost to on.
    (serf_cmds): Change add_pass to a take argv directive.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@759395 13f79535-47bb-0310-9956-ffa450edef68