SubEmitterSocket should apply listener with self

This commit makes `SubEmitterSocket.prototype.onmessage` apply its
listener method with `self` rather than `this`. This behaviour is
consistent with that of the `SubEmitter`, and provides access to the
underlying socket (rather than the message buffer) in the listener.

lib/sockets/sub-emitter.js

@@ -35,7 +35,7 @@ function SubEmitterSocket() {
  * @api private
  */
 
-SubEmitterSocket.prototype.onmessage = function(){
+SubEmitterSocket.prototype.onmessage = function(sock){
   var listeners = this.listeners;
   var self = this;
 
@@ -49,7 +49,7 @@ SubEmitterSocket.prototype.onmessage = function(){
       var m = listener.re.exec(topic);
       if (!m) continue;
 
-      listener.fn.apply(this, m.slice(1).concat(msg.args));
+      listener.fn.apply(self, m.slice(1).concat(msg.args));
     }
   }
 };