fasthttp 怎么添加headers

``` package main import ( "fmt" "time" "github.com/valyala/fasthttp" ) func main() { url := `http://localhost:81/page?key=123` req := fasthttp.AcquireRequest() resp := fasthttp.AcquireResponse() cnt := 0 defer func() { fasthttp.ReleaseResponse(resp) fasthttp.ReleaseRequest(req) }() //req.Header.SetContentType("application/json") req.Header.SetMethod("POST") req.SetRequestURI(url) requestBody := []byte(`{"request":"test"}`) req.SetBody(requestBody) for { if cnt > 10 { break } if err := fasthttp.Do(req, resp); err != nil { fmt.Println("请求失败:", err.Error()) return } b := resp.Body() fmt.Println("result:\r\n", string(b)) time.Sleep(3 * time.Second) cnt += 1 } } ``` 比如添加个ua怎么写？