code: public function add_header($name, $value) {
          $name = strtolower($name);
          $this->_headers[$name][] = $value;
          return $this;
      }
docs: Adds a new header.
  If there is already a header with the same name, the new one will
  be appended to it instead of replacing it.
  @param string $name the name of the header
  @param string $value the value of the header
  @return static the collection object itself