如何使用Golang http.ResponseWriter将HTML字符串显示为网页？

I'm following along the example in Chapter 19 from The Way to Go.

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.

go version go1.6.2 linux/amd64

How do I modify it to make it render as a form in the browser? Thanks.