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 - Ok that is how I read it too.  Seems to only apply when converting from HTTP/2 to HTTP/1.x.  I think we should have that covered in the `DefaultHttpHeaders` class.  If there are holes I think they can be patched in this class also.