Cakephp Auth login REST response

public function login()
{
    if($this->request->is('post')){
        if($this->Auth->login()){
            var_dump('Logged in successfully');

        }else{
            var_dump('Log in failed');
        }
    }
}

When I'm writing the above code, I'm getting output as following in browser view:

string(22) "Logged in successfully"

But when I'm rewriting the code as following to use as REST api:

public function login()
{
    if($this->request->is('post')){
        if($this->Auth->login()){
            $message = 'Logged in successfully';

        }else{
            $message = 'Error. Log in failed. Please, try again.';
        }
        $this->set(array(
            'message' => $message,
            '_serialize' => array('message')
        ));
    }
}

Now I'm getting response like the following:

{
   message: "Error. Log in failed. Please, try again."
}

I'm using 'Advanced Rest Client' extension for Chrome to check the REST response.