WebsocketProvider - fixed incorrect variable name

lib/websocket.js

@@ -71,7 +71,7 @@ WebSocketProvider.prototype.unload = function() {
     this.ws.close();
 };
 Object.defineProperty(WebSocketProvider.prototype, "onmessage", {
-    set: function(provider) { this.onMessage(provider); }
+    set: function(handler) { this.onMessage(handler); }
 });
 
 if (typeof(module) !== "undefined")