Laravel 4 如何将所有字段的所有验证错误消息作为 JSON struct 返回

I'm upgrading from Laravel 3 to Laravel 4. My app has some AJAX-only forms that are rendered client-side (i.e. there are no server-side views). Therefore, instead of passing validation errors to views with the withErrors() method, I've been returning the validation error objects to the client as JSON structures.

在《拉威尔3》中，我有这样一个故事:

$validation = Validator::make(Input::all(), $rules);
if($validation->fails())
{
  return json_encode($validation->errors);
}
//else handle task

但在Laravel 4中，错误消息受到保护:

$validation = Validator::make(Input::all(), $rules);
if($validation->fails())
{
  var_dump($validation->messages());
  return json_encode($validation->messages());
}
//else handle task

var_dump($validation->messages()) returns:

object(Illuminate\Support\MessageBag)[333]
  protected 'messages' => 
    array (size=1)
      'delete_confirm_password' => 
        array (size=1)
          0 => string 'The delete confirm password field is required.' (length=46)
  protected 'format' => string ':message' (length=8)

json_encode($validation->messages) returns

{}

Question:如何在Laravel 4中以JSON struct 返回所有字段的所有验证错误消息？