code: func (w *QWriter) U(s string) {
  	bb, ok := w.w.(*ByteBuffer)
  	if ok {
  		bb.B = appendURLEncode(bb.B, s)
  	} else {
  		w.b = appendURLEncode(w.b[:0], s)
  		w.Write(w.b)
  	}
  }
docs: // U writes url-encoded s to w.