docs: function XHR(opts){
    Polling.call(this, opts);
  
    if (global.location) {
      var isSSL = 'https:' == location.protocol;
      var port = location.port;
  
      // some user agents have empty `location.port`
      if (Number(port) !== port) {
        port = isSSL ? 443 : 80;
      }
  
      this.xd = opts.hostname != global.location.hostname ||
        port != opts.port;
    }
  }