Go HTTP サーバーが遅延後にチャンク データのみを送信するのはなぜですか? Content-Length を 0 に設定するにはどうすればよいですか?

package main

import (
    "fmt"
    "io"
    "log"
    "net/http"
    "time"
)

func main() {
    http.HandleFunc("/test", HandlePost)
    log.Fatal(http.ListenAndServe(":8080", nil))
}

func HandlePost(w http.ResponseWriter, r *http.Request) {
    flusher, ok := w.(http.Flusher)
    if !ok {
        panic("expected http.ResponseWriter to be an http.Flusher")
    }
    w.Header().Set("X-Content-Type-Options", "nosniff")

    ticker := time.NewTicker(time.Second)
    go func() {
        for t := range ticker.C {
            io.WriteString(w, "Chunk")
            fmt.Println("Tick at", t)
            flusher.Flush()
        }
    }()
    time.Sleep(time.Second * 5)
    ticker.Stop()
    fmt.Println("Finished: should return Content-Length: 0 here")
    w.Header().Set("Content-Length", "0")
}

$ telnet localhost 8080
Trying ::1...
Connected to localhost.
Escape character is '^]'.
GET / HTTP/1.1

HTTP/1.1 200 OK
Date: Tue, 02 Jun 2015 18:16:38 GMT
Content-Type: text/plain; charset=utf-8
Transfer-Encoding: chunked

9
Chunk #1

9
Chunk #2

...