url: a short host name + port is not a scheme

The function identifying a leading "scheme" part of the URL considered a few
letters ending with a colon to be a scheme, making something like "short:80"
to become an unknown scheme instead of a short host name and a port number.

Extended test 1560 to verify.

Reported-by: Hagai Auro
Fixes #3220
Closes #3223