fix(WebSocket): don't patch EventTarget methods twice

closes #235

On older versions of Chrome, patching via property descriptors isn't
available, but unlike Safari, window.EventTarget exists and is already
patched, so the EventTarget methods on WebSocket.prototype don't need to
be patched again.

Closes #233.

lib/patch/websocket.ts

@@ -3,7 +3,11 @@ import * as utils from '../utils';
 // we have to patch the instance since the proto is non-configurable
 export function apply() {
   var WS = (<any>global).WebSocket;
-  utils.patchEventTargetMethods(WS.prototype);
+  // On Safari window.EventTarget doesn't exist so need to patch WS add/removeEventListener
+  // On older Chrome, no need since EventTarget was already patched
+  if (!(<any>global).EventTarget) {
+    utils.patchEventTargetMethods(WS.prototype);
+  }
   (<any>global).WebSocket = function(a, b) {
     var socket = arguments.length > 1 ? new WS(a, b) : new WS(a);
     var proxySocket;