A new version of the spec came out last week Signing HTTP Messages v.02. It comes with a Diff from the previous version.
Btw. Since Authentication is not part of the IETF work, I have been working on how to integrate it back in to the Solid Project. Here is a very early spec which I worked from memory of a previous implementation. I was thinking of using your library to test out these ideas. Also: Is there a forum where people are discussing authentication with HTTP Sig?