title: Set Active Model serializer adapter per method
body: <p>I'm trying to build an api-only app in rails 5. I'm using active model serializer (<a href="https://rubygems.org/gems/active_model_serializers" rel="nofollow noreferrer">AMS</a>).
  On my <code>GET /users</code> (UsersController#index) endpoint, I want the root json to have a "users" keyword before the json array of users.</p>
  
  <p>If I set an initializer with <code>ActiveModelSerializers.config.adapter = :json
  </code> value, then I have the desired behavior. </p>
  
  <p>But then, on the  <code>GET /users/1</code> the simple user json comes under the key <code>user</code>. I'd like to get rid of this key (in this case) and simply answer the user json as the root json.</p>
  
  <p>The question is: how can I define the AMS adapter on a specific endpoint/response/method? Is that possible?</p>