Automatic Allow: header insertion

[idubrov]

Currently, allow() callback in UA returns list of allowed methods for particular request.

The problem is that, as per 13.2.1, UAC should provide Allow header in the INVITE requests initiated by it. It seems to make sense to gather that information automatically from the callback. However, it is not clear: 1) should we have multiple callback functions? 2) if not, should we have consistent behaviour for both outgoing requests/responses? 3) if yes, what should we pass to the callback then? URI (which will be from Request-URI for UAS case and Contact for UAC case)?