Why does this GoLang Mock HTTP responder return the wrong number of times it was called?

I'm writing test cases for my Go application that makes HTTP requests.
To simulate a response from the remote host, I have created this class stringProducer

type stringProducer struct {
	strings   []string
	callCount int
}

func (s *stringProducer) GetNext() string {
	if s.callCount >= len(s.strings) {
		panic("ran out of responses")
	}
	s.callCount++
	fmt.Println("s.CallCount = ", s.callCount)
	return s.strings[s.callCount-1]
}

func mockHTTPResponder(producer stringProducer) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
		w.Write([]byte(producer.GetNext()))
	})
}

Here is how I call it in my main function:

func main() {
	producer := stringProducer{
		strings: []string{"Hello World!"},
	}

	srv := httptest.NewServer(mockHTTPResponder(producer))
	if producer.callCount != 0 {
		panic("callCount is not 0")
	}

	var buf io.ReadWriter
	req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, fmt.Sprintf("%s/path/to/something", srv.URL), buf)

	newClient := http.Client{}

	newClient.Do(req)

	if producer.callCount != 1 {
		panic("callCount is not 1")
	}
}

In this code, when an HTTP request is made, it goes to the responder above which responds with some pre-specified text. It also causes the counter stringProducer.callCount to be incremented by 1.

From the program's output below, you can see that it prints a line showing that callCount was incremented to 1. However, when I check that same value, it is not 1. It is zero. Why? And how to fix that?

s.CallCount =  1
panic: callCount is not 1

goroutine 1 [running]:
main.main()
    /tmp/sandbox3935766212/prog.go:50 +0x118

Go Playground link here: https://play.golang.org/p/mkiJAfrMdCw

You pass by value stringProducer in mockHTTPResponder. When you do this you get a copy of the variable inside mockHTTPResponder. And all the following changes are made on that copy (original stringProducer are left unchanged):

func mockHTTPResponder(producer stringProducer) http.Handler { // <- producer is a copy of the original variable
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.WriteHeader(http.StatusOK)
        w.Write([]byte(producer.GetNext()))  // <- s.callCount++ on the copy
    })
}

Pass a pointer inside mockHTTPResponder.