protected function _storeHeader($headerName, $value, $append = false)
    {
        if (isset($this->_headers[$headerName])) {
            $this->_headers[$headerName][] = $value;
        } else {
            $this->_headers[$headerName] = array($value);
        }

        if ($append) {
            $this->_headers[$headerName]['append'] = true;
        }

    }