[xmpp] 3920: id attribute on error replies

[Matthew Wild <mwild1 at gmail.com>]

Minor issue in 3920. I think it would be helpful to add in section 9.3
that when generating error replies in response to stanzas, the
original stanza's 'id' attribute SHOULD be included in the reply.

For example we already have this text in 9.1.3:

   The 'id' attribute is used by the entity that generates a stanza
   ("the originating entity") to track any response or error stanza that
   it might receive in relation to the generated stanza from another
   entity (such as an intermediate server or the intended recipient).

But in truth there is no way (purely by the specs) to be sure error
replies will contain the correct id. I'd hope that most
implementations do the right thing (since this is common sense) but it
makes sense to change it for the future.

Agreed?

Matthew