Expose collection of items of different types without keys

Hi.

I've recently tried to expose collection of objects of different types, but it seems that it's not possible with a current version.

Here's the result I'm trying to achieve:

{
  included: [
    {
      id: 1,
      type: 'posts',
      attributes: {
        title: 'Title 1',
        date: '2015-05-01'
      }
    },
    {
      id: 2,
      type: 'posts',
      attributes: {
        title: 'Title 2',
        date: '2015-05-02'
      }
    },
    {
      id: 1,
      type: 'users',
      attributes: {
        first_name: 'John',
        last_name: 'Doe',
        email: 'john.doe@example.com'
      }
    }
  ]
}

And classes:

class Result < Grape::Entity
  expose :included, documentation: { is_array: true } do
    expose :posts, using: Post
    expose :user, using: User
  end
end

class Post < Grape::Entity
  expose :id
  expose :type
  expose :attributes do
    expose :title
    expose :date
  end
end

class User < Grape::Entity
  expose :id
  expose :type
  expose :attributes do
    expose :first_name
    expose :last_name
    expose :email
  end
end

Is there something I am missing?

Start by reproducing the Result, Post, and User exposure example from the issue and compare its output with the requested included array. Determine how heterogeneous collections without keys are intended to be represented, then verify that the resulting serialization contains both posts and users in the shown structure.