public function filterHeaders(&$headers) {
    $unset = array();
    foreach ($headers as $name => $value) {
      $lower = strtolower($name);
      if (isset($this->reservedHeaders[$lower])) {
        $unset[] = $name;
      }
    }
    foreach ($unset as $u) {
      unset($headers[$u]);
    }

    return $this;
  }