url: restore the functionality of 'curl -u :'

This commit fixes a regression introduced in
fddb7b44a79d78e05043e1c97e069308b6b85f79.

Reported by: Markus Moeller
Bug: http://curl.haxx.se/mail/archive-2013-06/0052.html
Merge pull request #2 from bagder/master

sync with upstream