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.
I'm not sure it is out of date and I think it will potentially create a stream if there is no stream associated with `streamDependency`.

See https://github.com/netty/netty/pull/3217/files#diff-9d991ca64301bcd5b3c00d053591f485R373.