core: call onsend_route for replies

- this feature is aimed at tracing replies with sip_trace; e.g.:
  onsend_route { sip_trace("sip:10.84.1.49:5060"); } would now trace the
  requests passing Kamailio, as well as the replies;
  This patch has been proposed earlier: http://lists.sip-router.org/pipermail/sr-users/2012-May/073250.html