func (h Html) render(htmlstr string, value_map interface{}, buf io.Writer) {
w := h.w
if buf == nil {
// To prevent headers from being sent too early.
buf = &bytes.Buffer{}
defer func(buf *bytes.Buffer) {
io.Copy(w, buf)
buf.Reset()
}(buf.(*bytes.Buffer))
}
t := html.Must(html.New("html").Funcs(w.HtmlFunc).Parse(htmlstr))
err := t.Execute(buf, value_map)
w.Check(err)
}