public function getHeaderRowHTML()
    {
        $output = [];
        foreach ($this->columns as $name => $column) {

            // If multiple formatters are defined, use the first for the header cell
            if (is_array($column)) {
                $column = $column[0];
            }

            if (!is_int($name)) {
                $field = $this->model->getElement($name);

                $output[] = $column->getHeaderCellHTML($field);
            } else {
                $output[] = $column->getHeaderCellHTML();
            }
        }

        return implode('', $output);
    }