[fix] Ensure valid status is passed to error handler (#622)

Closes #621

lib/transports/polling-xhr.js

@@ -257,7 +257,7 @@ Request.prototype.create = function () {
           // make sure the `error` event handler that's user-set
           // does not throw in the same tick and gets caught here
           setTimeout(function () {
-            self.onError(xhr.status);
+            self.onError(typeof xhr.status === 'number' ? xhr.status : 0);
           }, 0);
         }
       };