为什么设置Servlet setContextType不起作用？

Question

为什么设置setContextType不起作用？

我将setContextType设置为text/html;charset=UTF-8

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        PrintWriter out = response.getWriter();

        request.setCharacterEncoding("UTF-8");
        String username = request.getParameter("username");
        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");

        out.println(username);      
    }

但响应头为Content-Type: text/html;charset=ISO-8859-1