Golang - http.ResponseWriter.WriteHeader() caused deadlock

I was trying to use the httptest package in golang. I found out something I don't understand. Here is the code:

package main

import (
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
    "net/http/httptest"
)

func main() {
    ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello1"))
    }))
    ts.Close()
    ts = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello2"))
    }))
    ts.Close()
    ts = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.WriteHeader(100)
        w.Write([]byte("Hello3"))
    }))

    res, err := http.Get(ts.URL)
    if err != nil {
        log.Fatal(err)
    }
    greeting, err := ioutil.ReadAll(res.Body)
    res.Body.Close()
    if err != nil {
        log.Fatal(err)
    }

    ts.Close()
    fmt.Printf("%s", greeting)
}

In this code example, I was trying to open and close httptest servers several times. Somehow it caused deadlock The Go Playground. I tried on my own environment (go version go1.7.4 darwin/amd64) and it caused hanging without responding at all.

My question is: Why w.WriteHeader(100) caused deadlock but w.WriteHeader(200) doesn't? Is it the bug from the core library of Golang or just I misunderstood some usage? Tks!