HTTP/2 Draft 15

Motivation:
A new draft of the HTTP/2 spec has been released.

Modifications:
Make updates as defined in https://tools.ietf.org/html/draft-ietf-httpbis-http2-15

Result:
HTTP/2 codec is draft 15 compliant.
@normanmaurer @trustin - This was due to https://tools.ietf.org/html/draft-ietf-httpbis-http2-15#section-10.3.  Does this make sense?