rest.URLJoin resolves its path as a URL reference according to the URL
standard, so a path beginning with `//` is a network-path reference
which replaces the host (and port and userinfo) of the base URL. That
is often the wrong behaviour.
Add rest.URLJoinRoot which joins an unescaped root onto a base URL and
refuses any root which parses with a scheme, host or userinfo of its own.
Nick Craig-Wood
28562c8a79