[fix] Define "requestsCount" var and "requests" hash unconditionally (#490)

There are situations in which `global.document` can be defined after the file is loaded, but before a request is created.

Author: Aaron Carlino

lib/transports/polling-xhr.js

@@ -393,9 +393,10 @@ Request.prototype.abort = function () {
  * emitted.
  */
 
+Request.requestsCount = 0;
+Request.requests = {};
+
 if (global.document) {
-  Request.requestsCount = 0;
-  Request.requests = {};
   if (global.attachEvent) {
     global.attachEvent('onunload', unloadHandler);
   } else if (global.addEventListener) {