问使用Tablesorter从表头禁用上/下图标，而不是隐藏它

EN

thead th.up {
    padding-right: 20px;
    background-image: url(data:image/gif;base64,R0lGODlhFQAEAIAAACMtMP///yH5BAEAAAEALAAAAAAVAAQAAAINjI8Bya2wnINUMopZAQA7);
}
thead th.down {
    padding-right: 20px;
    background-image: url(data:image/gif;base64,R0lGODlhFQAEAIAAACMtMP///yH5BAEAAAEALAAAAAAVAAQAAAINjB+gC+jP2ptn0WskLQA7);
}

$('table').tablesorter({

    // customize header HTML
    onRenderHeader: function(index) {
        // the span wrapper is added by default
        this.wrapInner('<span class="icons"></span>');
    },
    cssAsc: 'up',
    cssDesc: 'down',
});