Update poll.js to include XML Header in fetch request

appengine/standard/iap/js/poll.js

@@ -1,11 +1,11 @@
-// Copyright Google Inc.
-//
+// Copyright 2017 Google LLC
+
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
-//
+
 //     http://www.apache.org/licenses/LICENSE-2.0
-//
+
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -54,10 +54,21 @@ function sessionRefreshClicked() {
 
 function checkSessionRefresh() {
   if (iapSessionRefreshWindow != null && !iapSessionRefreshWindow.closed) {
-    fetch('/favicon.ico').then(function(response) {
+    // Attempting to start a new session.
+    // XMLHttpRequests is used by the server to identify AJAX requests
+    fetch('/favicon.ico', {
+          method: "GET",
+          credentials: 'include',
+          headers: {
+              'X-Requested-With': 'XMLHttpRequest'
+          }
+    }).then(function(response) {
+      // Checking if browser has a session for the requested app
       if (response.status === 401) {
+        // No new session detected. Try to get a session again
         window.setTimeout(checkSessionRefresh, 500);
       } else {
+        // Session retrieved.
         iapSessionRefreshWindow.close();
         iapSessionRefreshWindow = null;
       }