Messages - 2.1.3 put reference to the registration.

[Nat Sakimura]

No description provided.

[Michael Jones]

What we want to reference is the The OAuth Authorization Endpoint Response Type Registry. But this won't exist until OAuth 2.0 becomes an RFC and IANA creates the registry. I suggest we put this one on hold until then.

[Michael Jones]

There's now a normative reference to the OAuth Response Types spec