Better logout

Hello Sau Sheong Chang,

Congratulations for the book, it's very useful.

The logout function in the book always triggers the warning and doesn't delete the browser cookie:

func logout(writer http.ResponseWriter, request *http.Request) {
	cookie, err := request.Cookie("_cookie")
	if err != http.ErrNoCookie {
		warning(err, "Failed to get cookie")
		session := data.Session{Uuid: cookie.Value}
		session.DeleteByUUID()
	}
	http.Redirect(writer, request, "/", 302)
}

A possible solution could be:

func logout(writer http.ResponseWriter, request *http.Request) {
	cookie, err := request.Cookie("_cookie")
	if err != http.ErrNoCookie {
		session := data.Session{Uuid: cookie.Value}
		session.DeleteByUUID()
		cookie.MaxAge = -1
		cookie.Expires = time.Unix(1, 0)
		http.SetCookie(writer, cookie)
	} else {
		warning(err, "Failed to get cookie")
	}
	http.Redirect(writer, request, "/", 302)
}