HTTP/2 Draft 16

Motivation:
HTTP/2 draft 16 has been released https://tools.ietf.org/html/draft-ietf-httpbis-http2-16.

Modifications:
The HTTP/2 codec should be updated to support draft 16.

Result:
HTTP/2 codec is draft 16 compliant.
The `connection.goAwaySent()` was redundant with the `shouldIgnoreFrame(...)` method.  I just re-factored the if clause in this method to (hopefully) remove the redundancy between checking this clause and a null stream check.  See https://github.com/netty/netty/pull/3217/files#diff-179f52a9de82166ed577701a1412b5a0R563.