Validate urls like http://.com or http://..com

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11702
| License       | MIT

This PR fixes the regular expression to accept valid URLs. It does not allow to start a domain name with "-" or "."
Yes, you're right. My example works though. But actually, something like ``http://co`` did work before (when validating with the plain HTTP scheme).