Enable definition of timeouts for xhr-polling

The definition of timeouts is desirable to cancel long outstanding
requests automatically via browser mechanisms. Specifically, it can
happen that lots of pending XHR requests are scheduled, but are never
purged by the browser due to the lack of a timeout definition.

lib/transports/polling-xhr.js

@@ -30,6 +30,7 @@ function empty () {}
 
 function XHR (opts) {
   Polling.call(this, opts);
+  this.requestTimeout = opts.requestTimeout;
 
   if (global.location) {
     var isSSL = 'https:' === location.protocol;
@@ -84,6 +85,7 @@ XHR.prototype.request = function (opts) {
   opts.ca = this.ca;
   opts.ciphers = this.ciphers;
   opts.rejectUnauthorized = this.rejectUnauthorized;
+  opts.requestTimeout = this.requestTimeout;
 
   // other options for Node.js client
   opts.extraHeaders = this.extraHeaders;
@@ -147,6 +149,7 @@ function Request (opts) {
   this.isBinary = opts.isBinary;
   this.supportsBinary = opts.supportsBinary;
   this.enablesXDR = opts.enablesXDR;
+  this.requestTimeout = opts.requestTimeout;
 
   // SSL options for Node.js client
   this.pfx = opts.pfx;
@@ -228,6 +231,10 @@ Request.prototype.create = function () {
       xhr.withCredentials = true;
     }
 
+    if (this.requestTimeout) {
+      xhr.timeout = this.requestTimeout;
+    }
+
     if (this.hasXDR()) {
       xhr.onload = function () {
         self.onLoad();