<!-- method="POST" POST 必须大写 -->
<form action="http://localhost:8999/hello" method="POST">
username <input type="text" name="username"/> <br>
password <input type="text" name="password" /> <br>
<input type="submit">
</form>
// 需要留意的以下问题
func handle(w http.ResponseWriter, r *http.Request) {
// method="POST" post 必须为大写, 否则 r.PostForm is nil
r.ParseForm()
fmt.Fprintln(w, "Form ", r.Form)
fmt.Fprintln(w, "PostForm", r.PostForm)
}