Q:

How to display HTML string as a web page using Golang http.ResponseWriter?

I'm following along the example in Chapter 19 from The Way to Go.
This is the content of my main.go
package main

import (
    "fmt"
    "net/http"
)

const AddForm = `
<form method="POST" action="/add">
URL: <input type="text" name="url">
<input type="submit" value="Add">
</form>
`

func main() {
    http.HandleFunc("/add", Add)
    http.ListenAndServe(":8080", nil)
}

func Add(w http.ResponseWriter, r *http.Request) {
    url := r.FormValue("url")
    if url == "" {
        fmt.Fprint(w, AddForm)
        return
    }
    key := "Placeholder"
    fmt.Fprintf(w, "http://localhost:8080/%s", key)
}

According to the book and its screenshot, I should see a valid form rendered in my browser. But right now, I can only see the raw string.
I'm very new to go, so I'm not sure if the language has evolved a lot since the example was written. My go version is go version go1.6.2 linux/amd64 and I think the book is written in 2012 with older version of go. 
How do I modify it to make it render as a form in the browser? Thanks.

A:

After reading through the doc, in current version of golang, http.ResponseWriter has a default Content-type as text/plain, and after manually set the content type as text/html, things are working as expected. 
if url == "" {
    w.Header().Set("Content-Type", "text/html; charset=utf-8")
    fmt.Fprint(w, AddForm)
    return
}

