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.
@nmittler - The intention is just to apply this during conversion to HTTP/1.x headers.  I don't think the HTTP/2 framing layers have the same restrictions?  I think I'm missing something here.