title: Getting the active_model_serializer for model
body: <p>I am trying to send multiple different objects over a generic request
  - more specificly - I have different types of users, with different model, all might be return when creating a new session.
  looking at rail casts #409 - ryan calls model.active_model_serializers.new - this does not work for me.
  is there any other way to get the seriailzer without knowing what model am I serializing? </p>
  
  <p>I am using rails 4.2 and active_model_serializers 0.9.3</p>