Relax URI validation.

URIs are no longer validated against a scheme by default. This allows
usage of path only URI for example. To validate against a scheme or
authority `url-authority` and `url-schemes` can be used.
Nothing. I think URL validation is very messy and using test.check will drive you insane, if you are not building something from scratch and follow the RFC. This would require a proper parser and some free time ;)