highchart tooltip useHTML does not work

[seanlook]

BarChart:

        options={
        'tooltip': {
            'shared': True,
            'useHTML': True,
            'headerFormat': '<span style="font-size:10px">{point.key}</span><table>',
            'pointFormat': '<tr><td style="color:{series.color};padding:0">{series.name}: </td>'
                           '<td style="padding:0"><b>{point.y:.1f} mm</b></td></tr>',
            'footerFormat': '</table>'
        },
    }

useHTML attribute does not work in html page. I saw '<' is escaped.
After I remove 'cls=JSONEncoderForHTML' params from highcharts.py line:179 , it works.