Add messageId to the final object

I'm not sure if this make sense to you, but it's very useful to have the (unquoted) message-id header inside as a property on the object, first of all because (as described in [RFC 2822](http://www.ietf.org/rfc/rfc2822.txt)) message-id is semantically related to 'in-reply-to' and 'references' fields.