Write the headers row

        :param obj sheet: an odswriter Sheet object

def _render_headers(self, sheet):
        """
        Write the headers row

        :param obj sheet: an odswriter Sheet object
        """
        headers = getattr(self, 'headers', ())
        labels = [header['label'] for header in headers]
        extra_headers = getattr(self, "extra_headers", ())
        labels.extend([header['label'] for header in extra_headers])
        sheet.writerow(labels)