Failing Go + Socket.io CORS server

gistfile1.txt

package main

 import (
         "log"         
         "net/http"
         "github.com/googollee/go-socket.io"
 )

 func SayHelloWorld(w http.ResponseWriter, r *http.Request) {
         w.Write([]byte("Hello, World!"))        
 }

 type CrossOriginServer struct {}

 func (s *CrossOriginServer) ServeHTTP(rw http.ResponseWriter, req *http.Request) {     
     allowHeaders := "Accept, Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization"
     if origin := req.Header.Get("Origin"); origin != "" {
         rw.Header().Set("Access-Control-Allow-Origin", origin)
         rw.Header().Set("Access-Control-Allow-Credentials", "true") // Without this, error: Credentials flag is 'true', but the 'Access-Control-Allow-Credentials' header is ''. It must be 'true' to allow credentials.
         rw.Header().Set("Access-Control-Allow-Methods", "POST, PUT, PATCH, GET, DELETE")
         rw.Header().Set("Access-Control-Allow-Headers", allowHeaders)
     }
     if req.Method == "OPTIONS" {
         return
     }

     server, err := socketio.NewServer(nil)
       if err != nil {
           log.Fatal(err)
       }
     server.On("connection", func(so socketio.Socket) {

        log.Println("on connection")
   
        so.On("disconnection", func() {
            log.Println("on disconnect")
        })
     })
     server.On("error", func(so socketio.Socket, err error) {
        log.Println("error:", err)
     })    

     mux := http.NewServeMux()
     mux.Handle("/socket.io/", server)
     mux.HandleFunc("/", SayHelloWorld)

     mux.ServeHTTP(rw, req)
 }

 func main() {
   log.Println("listening on :8080")
   http.ListenAndServe(":8080", &CrossOriginServer{})
 }

I am able to see "on connect" and "on disconnect" console log messages in the Go server, but in the client:

WebSocket connection to 'wss://api.domain.com/socket.io/?EIO=3&transport=websocket&sid=Pp_I3GaGUDpS9TBCeAVj' failed: Error during WebSocket handshake: Unexpected response code: 400 

api.domain.com/socket.io/?EIO=3&transport=polling&t=1420716581398-1473&sid=Pp_I3GaGUDpS9TBCeAVj:1 POST https://api.optkit.com/socket.io/?EIO=3&transport=polling&t=1420716581398-1473&sid=Pp_I3GaGUDpS9TBCeAVj 400 (Bad Request) 

(sometimes 2 or 3 of these per one error above)

Headers:

HTTP/1.1 400 Bad Request
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: Accept, Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization
Access-Control-Allow-Methods: POST, PUT, PATCH, GET, DELETE
Access-Control-Allow-Origin: http://fiddle.jshell.net
Content-Type: text/plain; charset=utf-8
Date: Thu, 08 Jan 2015 11:44:36 GMT
Content-Length: 12
Connection: close