Set yourself up an http.Server example, telnet to it and issue a GET with a double-slash
in the lead (or just visit in in your browser with a double-slash URL such as
http://myhost//myfile). It will redirect you to / instead of to /myfile. /// or ////
redirect as expected however.

The problem seems to be in ParseURL:
        // Maybe path is //authority/path
        if len(path) > 2 && path[0:2] == "//" {
                url.Authority, path = split(path[2:], '/', false)
        }