title: index method in controller not working for active model serializer
body: <p>I want to send a <code>json</code> response containing a collection of my <code>bookmarks</code> resource. For some reason active model serializer is just not manipulating my json as expected. It's only rendering the default response.</p>
  
  <p>Here is my <code>BookmarksSerializer</code> serializer in <code>app/serializers/bookmarks_serializer.rb</code></p>
  
  <pre><code>class BookmarksSerializer &lt; ActiveModel::Serializer
    attributes :id, :url, :title
  end
  </code></pre>
  
  <p>and my controller method inside my <code>Api::UsersController</code>which has a <code>respond_to :json</code> filter on top</p>
  
  <pre><code>def index
    user = User.find_by(username: params[:username])
    @bookmarks = user.bookmarks
    render json: @bookmarks.to_json
  end
  </code></pre>
  
  <p>Instead of getting only :id, :title and :url, I am getting the full default hash</p>
  
  <p>What am I doing wrong? I am using <code>rails 4.1.5</code> and the github version of the <code>active_model_serializers</code>
  gem.</p>
  
  <pre><code>gem 'active_model_serializers', github: 'rails-api/active_model_serializers'
  </code></pre>
  
  <p>FYI: I have a show method inside the same controller and it's serializer picks up the single resource just fine. It's just the collection in the index method that is not rendering. Please help</p>