Http Over TLS Golang Not Receiving Headers

I am sending 'User-Agent Header' from my client but not receiving on server-side but when i use browser it works.

server code :

package main

import (
  "crypto/tls"
  "fmt"
  "log"
  "net/http"
  "time"
)

const (
  defaultServerDomain = "localhost:443"
  certPemPath = "./selfsigned.crt"
  privKeyPath = "./selfsigned.key"
)

func main() {
  mux := http.NewServeMux()
  mux.HandleFunc("/demo/echo", func(w http.ResponseWriter, req *http.Request) {
    w.Header().Add("Strict-Transport-Security", "max-age=63072000; includeSubDomains")
    w.Write([]byte("This is an example server.\n"))
    fmt.Printf("\n\nUserAgent : %s", req.UserAgent())

  })
  cfg := &tls.Config{
    MinVersion:               tls.VersionTLS12,
    CurvePreferences:         []tls.CurveID{tls.CurveP521, tls.CurveP384, tls.CurveP256},
    PreferServerCipherSuites: true,
    CipherSuites: []uint16{
      tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
      tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
      tls.TLS_RSA_WITH_AES_256_GCM_SHA384,
      tls.TLS_RSA_WITH_AES_256_CBC_SHA,
    },
  }
  srv := &http.Server{
    ReadHeaderTimeout: 2 * time.Second,
    Addr:              defaultServerDomain,
    Handler:           mux,
    TLSConfig:         cfg,
    TLSNextProto:      make(map[string]func(*http.Server, *tls.Conn, http.Handler), 0),
  }
  log.Fatal(srv.ListenAndServeTLS(certPemPath, privKeyPath))
}

client code :

package main

import (
  "crypto/tls"
  "crypto/x509"
  "fmt"
  "io"
  "log"
  "net/http"
  "os"
  "time"
)

const (
  serverDomain = "https://localhost:443/demo/echo"
  certPemPath  = "./selfsigned.crt"
)

func main() {

  cert, err := os.ReadFile(certPemPath)
  if err != nil {
    log.Fatal(err)
  }
  certPool := x509.NewCertPool()
  if ok := certPool.AppendCertsFromPEM(cert); !ok {
    log.Fatalf("unable to parse cert from %s", certPemPath)
  }

  client := &http.Client{
    Timeout: 2 * time.Second,
    Transport: &http.Transport{
      DisableKeepAlives:  false,
      TLSClientConfig: &tls.Config{
        RootCAs: certPool,
      },
    },
  }

  r, err := http.NewRequest("GET", serverDomain, nil)
  req, err := client.Do(r)
  if err != nil {
    log.Fatal(err)
  }
  req.Header.Set("User-Agent", "Mozilla/5.0 (Windows NT 10.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.7113.93 Safari/537.36")
  defer req.Body.Close()

  html, err := io.ReadAll(req.Body)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Printf("%v\n", req.Status)
  fmt.Printf(string(html))
}

Suppose to get server-side :

UserAgent : Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0 Waterfox/91.4.2

Instead i get this :

UserAgent : Go-http-client/1.1

But when i use browser in my case waterfox then it works i get

UserAgent : Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0 Waterfox/91.4.2

Is it something to do with code or certificate i am using selfsinged certificate