[fix] Trigger xhr polling transport unload handler on 'pagehide' or '…

…unload' instead of 'beforeunload' (#604)

...to prevent broke connection on cancelled beforeunload event

Fixes #603

lib/transports/polling-xhr.js

@@ -401,7 +401,8 @@ if (typeof document !== 'undefined') {
   if (typeof attachEvent === 'function') {
     attachEvent('onunload', unloadHandler);
   } else if (typeof addEventListener === 'function') {
-    addEventListener('beforeunload', unloadHandler, false);
+    var terminationEvent = 'onpagehide' in self ? 'pagehide' : 'unload';
+    addEventListener(terminationEvent, unloadHandler, false);
   }
 }