title: Serialize relationships with custom fields in ActiveModelSerializer
body: <p>I'm having a bit of trouble using ActiveModelSerializers to serialize my model correctly. The setup is as follows:</p>
  
  <pre><code># controllers/posts_controller.rb
  class PostsController &lt; ApplicationController
  
    def index
     render json: @posts, each_serializer: PostSerializer, include: %w(comments), fields: post_fields
    end
  
    private
  
    def post_fields
      { posts: [:name, :content, :author] }
    end
  end
  
  # serializers/post_serializer.rb
  class PostSerializer &lt; ActiveModelSerializer
    attributes :name, :content, :author
    has_many :comments, serializer: CommentSerializer
  end
  </code></pre>
  
  <p>When I make a request to the <code>posts#index</code> endpoint, I'm expecting a JSON response that is formatted to the JSON-API spec, like the following:</p>
  
  <pre><code>{
    "data": {
      "type": "post",
      "id": 1,
      "attributes": {
        "name": "My first post",
        "author": "Mr. Glass",
        "content": "This is my first post ..."
      },
      "relationships": {
        "comments": {
          "data": {
            "id": 1,
            "type": "comments"
          }
        }
      }
    },
    "included": {
      "type": "comments",
      "id": 1,
      "attributes": {
        "content": "First!"
      }
    }
  }
  </code></pre>
  
  <p>However, the actual response is this:</p>
  
  <pre><code>{
    "data": {
      "type": "post",
      "id": 1,
      "attributes": {
        "name": "My first post",
        "author": "Mr. Glass",
        "content": "This is my first post ..."
      }
    },
    "included": {
      "type": "comments",
      "id": 1,
      "attributes": {
        "content": "First!"
      }
    }
  }
  </code></pre>
  
  <p>And the whole relationship block is just missing. Is there a way to get the relationship block in the actual response to show up again?</p>