http.go
func (req *Request) AppendBodyString(s string) {
	req.RemoveMultipartFormFiles()
	req.closeBodyStream()
	req.bodyBuffer().WriteString(s)
}
func (req *Request) AppendBodyString(s string) {
	req.RemoveMultipartFormFiles()
	req.closeBodyStream()
	req.bodyBuffer().WriteString(s)
}
// AppendBodyString appends s to request body.