mux := http.NewServeMux()
mux.HandleFunc("/me", func(w http.ResponseWriter, req *http.Request) {
  w.Header().Set("Content-Type", "text/plain; charset=utf-8")
  w.WriteHeader(200)
  fmt.Fprintf(w, "Request OK")
})

transport := gomock.NewTransport()
transport.Stub("graph.facebook.com", mux)

client := &http.Client{
  Transport: transport,
}

resp, err := client.Get("https://graph.facebook.com/me")
if err != nil {
  log.Fatal(err)
}

body, err := ioutil.ReadAll(resp.Body)
if err != nil {
  log.Fatal(err)
}
fmt.Println(string(body))
// Output: Request OK

mux := http.NewServeMux()
mux.HandleFunc("/me", func(w http.ResponseWriter, req *http.Request) {
  w.Header().Set("Content-Type", "text/plain; charset=utf-8")
  w.WriteHeader(200)
  fmt.Fprintf(w, "Request OK")
})

transport := gomock.NewTransport()
transport.Stub("graph.facebook.com", mux)

http.DefaultTransport = transport
defer gomock.ResetDefaultTransport()

client := &http.Client{}

resp, err := client.Get("https://graph.facebook.com/me")
if err != nil {
  log.Fatal(err)
}

body, err := ioutil.ReadAll(resp.Body)
if err != nil {
  log.Fatal(err)
}

fmt.Println(string(body))
// Output: Request OK