Consider upgrading HttpUrl to IDNA 2008

[swankjesse]

We're currently using java.net.IDN which implements IDNA 2003 on the systems I tried.
http://www.unicode.org/reports/tr46/

I actually wrote & discarded a nearly-complete implementation here:
#1612

The PR does mapping, but not punycode or deviants.

[yschimke]

IDN hasn't taken over, we don't see enough usage to justify owning custom implementation, so closing. Following the JVM and Android platform.

Won't fix.