Support for draft -05 and above

[jricher]

I'm one of the authors of the HTTP Message Signatures spec, and I was pointed to this implementation. I wanted to let you know that there have been several major revisions since -00, and now that the major churn has settled down we'd love to see support for the updated spec.

https://www.ietf.org/archive/id/draft-ietf-httpbis-message-signatures-05.html