Ensure opaque URL paths always roundtrip

[annevk]

See whatwg/url#844.

[anonrig]

I see that this has not yet merged. Is there a test that was not passing, and now has to pass due to this change? Would you mind sharing, so we can validate correctness of Ada? @annevk

[annevk]

See whatwg/url#784 (comment). I need some firmer commitments from browsers to continue with this.

[annevk]

This has now landed with a slightly different approach.