Fixed #18314 -- request.build_absolute_uri() not handling paths starting with //

``HttpRequest.build_absolute_uri()`` now correctly handles paths starting with ``//``.
``WSGIRequest`` now doesn't remove all the leading slashes either,
because ``http://test/server`` and http://test//server`` aren't the same thing
(RFC2396).
build_absolute_uri() (wouldn't hurt ot make it a link)