The http2.Transport was able to send bogus header keys & values.
This changes rejects them earlier, before they hit the wire.

In the process, mirror the lexical rules from the http package to x/net.
Maintaining two copies has gotten increasingly annoying.

Updates golang/go#14048

Change-Id: I20abcdeea92e7dc8706a1bbd60688ee8843a2b12
Reviewed-on: https://go-review.googlesource.com/23229Reviewed-by: Andrew Gerrand <adg@golang.org>