Auth-Info purpose consistency

[hruellan]

I think the purpose of the headers should be made more consistent across the document.
In the Introduction, they are used to "return additional information during or after authentication", while in 3, the Authentication-Info header is used to "communicate additional information regarding the successful authentication".

DIGEST use it in an optional manner, to convey additional information after a successful authentication.
Scram is using it in a mandatory manner, to finalize the authentication, by conveying information for authenticating the server.

I think that Authentication-Info should be used by the server once the client is authenticated (i.e. the status code is not 401), to either convey additional information or finalize the authentication.

Proposal in #47.

[reschke]

This change was applied in 849702e