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.
@Scottmitch no I think this is now a requirement of the HTTP/2 framing layer itself.  The spec now mandates the headers that are allowed to be passed on the wire.  The purpose is for translation between HTTP/2 and HTTP/1.1, but it's still a framing requirement.