schemes: fix // breakage with Python 2.6.5 (
issue2111)
Recent Pythons (e.g. 2.6.5 and 3.1) introduce a change that causes
urlparse.urlunparse(urlparse.urlparse('x://')) to return 'x:' instead of 'x://'i and
urlparse.urlunparse(urlparse.urlparse('x:///y')) to return 'x:/y' instead of 'x:///y'.
Fix url.hidepassword() and url.removeauth() to handle these cases.