CORS issue about go-socket.io HOT 6 CLOSED

func (s *customServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Access-Control-Allow-Credentials", "true")
    origin := r.Header.Get("Origin")
    w.Header().Set("Access-Control-Allow-Origin", origin)
    s.Server.ServeHTTP(w, r)
}

package main

import (
    "log"
    "net/http"

    "github.com/googollee/go-socket.io"
    "github.com/rs/cors"
)

func main() {
    mux := http.NewServeMux()
    mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Content-Type", "application/json")
        w.Write([]byte("{\"hello\": \"world\"}"))
    })

    // cors.Default() setup the middleware with default options being
    // all origins accepted with simple methods (GET, POST). See
    // documentation below for more options.
    server, err := socketio.NewServer(nil)
    if err != nil {
        log.Fatal(err)
    }
    server.On("connection", func(so socketio.Socket) {
        so.Emit("chat", "hello message")
        log.Println("on connection")
        so.On("chat", func(msg string) {
            log.Println("recieved message", msg)
        })
        // Socket.io acknowledgement example
        // The return type may vary depending on whether you will return
        // For this example it is "string" type
        so.On("chat message with ack", func(msg string) string {
            return msg
        })
        so.On("disconnection", func() {
            log.Println("disconnected from chat")
        })
    })
    server.On("error", func(so socketio.Socket, err error) {
        log.Println("error:", err)
    })

    mux.Handle("/socket.io/", server)
    mux.Handle("/assets", http.FileServer(http.Dir("./assets")))

       // provide default cors to the mux
    handler := cors.Default().Handler(mux)

c := cors.New(cors.Options{
        AllowedOrigins:   []string{"*"},
        AllowCredentials: true,
    })

    // decorate existing handler with cors functionality set in c
    handler = c.Handler(handler)

    log.Println("Serving at localhost:5000...")
    log.Fatal(http.ListenAndServe(":5000", handler))
}

type customServer struct {
  Server *socketio.Server
}