remove invalid value for strict mode (#344)

Author: nkzawa

lib/transports/polling.js

@@ -20,7 +20,7 @@ module.exports = Polling;
 
 var hasXHR2 = (function() {
   var XMLHttpRequest = require('xmlhttprequest');
-  var xhr = new XMLHttpRequest({ agent: this.agent, xdomain: false });
+  var xhr = new XMLHttpRequest({ xdomain: false });
   return null != xhr.responseType;
 })();