sip_session: minor change

darcs-hash:20090330120806-b2873-f946499ba4ff3eaf38a5c293738b6b4ae5068719
sip_session: use 'echo' argument to recv_char

darcs-hash:20090330120825-b2873-f53dae4a92bb9d2e1fa9e8aa0092b35401b599bb