title: Active Model Serializer not working with json_api adapter
body: <p>I am trying to use custom serializers for the relationships in a serializer and the json_api adapter enabled. However the relationships are not serialized correctly (or, better, not at all displayed/serialized). </p>
  
  <p><strong>PostController.rb</strong></p>
  
  <pre><code>def index
    render json: Post.all, each_serializer: Serializers::PostSerializer
  end
  </code></pre>
  
  <p><strong>Serializer</strong></p>
  
  <pre><code>module Api
    module V1
      module Serializers
        class PostSerializer &lt; ActiveModel::Serializer
          attributes :title, :id
  
          belongs_to :author, serializer: UserSerializer
          has_many :post_sections, serializer: PostSectionSerializer
        end
      end
    end
  end
  </code></pre>
  
  <p><strong>JSON output:</strong> </p>
  
  <pre><code>{
      "data": [
          {
              "attributes": {
                  "title": "Test Title"
              },
              "id": "1",
              "relationships": {
                  "author": {
                      "data": {
                          "id": "1",
                          "type": "users"
                      }
                  },
                  "post_sections": {
                      "data": [
                          {
                              "id": "1",
                              "type": "post_sections"
                          }
                      ]
                  }
              },
              "type": "posts"
          }
      ]
  }
  </code></pre>
  
  <p>As you can see, the relationships are not fulfilled, which happens <strong>only if I specify a custom serializer</strong> for the relationships!!</p>
  
  <p>If I do something like this: </p>
  
  <pre><code>module Api
    module V1
      module Serializers
        class PostSerializer &lt; ActiveModel::Serializer
          attributes :title, :id
  
          belongs_to :author # no custom serializer!
          has_many :post_sections # no custom serializer!
        end
      end
    end
  end
  </code></pre>
  
  <p>The relationships are shown correctly, but not using a custom serializer... 
  What's the issue here ? </p>
  
  <h1>EDIT</h1>
  
  <p>According to the <a href="http://jsonapi.org/format/#document-resource-identifier-objects" rel="nofollow">json API 1.0 Format</a>, what I am getting back is the so-called <code>resource identifier object</code>. </p>
  
  <blockquote>
    <p>The following primary data is a single resource identifier object that
    references the same resource:</p>
    
    <p>{   "data": {
        "type": "articles",
        "id": "1"   } }</p>
  </blockquote>
  
  <p>Is there a way to prevent getting resource identifier objects, and get the actual data instead ? </p>