reading websocket data returns random bytes

[Prof1-web]

Script:

package main

import (
    "fmt"
    "io"
    "log"
    "net"

    "github.com/gobwas/ws"
)

func HandleConn(conn net.Conn) {
    for {
        header, err := ws.ReadHeader(conn)
        if err != nil {
            log.Fatal(err)
        }

        buf := make([]byte, header.Length)
        _, err = io.ReadFull(conn, buf)
        if err != nil {
            log.Fatal(err)
        }
        fmt.Println(buf)
        fmt.Println(string(buf))
    }
}

func main() {
    ln, err := net.Listen("tcp", "localhost:8080")
    if err != nil {
        log.Fatal(err)
    }
    for {
        conn, err := ln.Accept()
        if err != nil {
            log.Fatal(err)
        }
        _, err = ws.Upgrade(conn)
        if err != nil {
            log.Fatal(err)
        }
        go HandleConn(conn)
    }
}

I do in browser console:

let socket = new WebSocket("ws://127.0.0.1:8080")
socket.send("Hello world")`

I see random bytes in the my terminal. Each call to socket.send("Hello world") return different bytes. But the length of the byte array is always equal to the length of the string. How can I fix this?