IDNA hostnames end up using emdash instead of -- for ACE-encoded labels

[sleevi]

https://url.spec.whatwg.org/commit-snapshots/0ce82da2b6734cdfa55652954ecc9dee9ed9b951/#example-host-psl

The public suffix renders as xn-kgbechtv when it should be xn--kgbechtv

[annevk]

This happened in #391 and nobody noticed at the time. (It also seems to be a hyphen.)

[annevk]

See #472.