server.go
func (ctx *RequestCtx) WriteString(s string) (int, error) {
	ctx.Response.AppendBodyString(s)
	return len(s), nil
}
func (ctx *RequestCtx) WriteString(s string) (int, error) {
	ctx.Response.AppendBodyString(s)
	return len(s), nil
}
// WriteString appends s to response body.