Return the polyline of the activity with the given id.

        :param style: The type of polyline to return. May be one of
                      'google', 'svg', or 'geojson'.

def get_polyline(self, id_num, style='google'):
        """Return the polyline of the activity with the given id.

        :param style: The type of polyline to return. May be one of
                      'google', 'svg', or 'geojson'.

        """
        parts = ['my', 'activities', id_num, 'polyline']
        if style != 'google':
            parts.append(style)
        url = self._build_url(*parts)

        return self._json(url)