The transaction matching logic failed on server side when the To URI was
changed between INVITE and ACK. Now it is possible to change the To/From URI
in nua_ack(), too.

Binary API was changed, too, and accessor function nta_outgoing_branch() was
added.

darcs-hash:20070502151640-55b16-73889a36bd0cb0d74413bb258e01c46193748b8f.gz