Serialize polymorphic, embed-in-root relationships by class name instead of association name

Since entities are embedded at the root level, polymorphic associations should use the class name under polymorphism instead of the association name. `has_one` and `has_many` relationships have been updated to reflect this, and `DefaultSerializer` now represents polymorphic types just as `Serializer` would.

This change brings active_model_serializers much closer to supporting Ember Data sideloading with polymorphism and the `ActiveModelAdapter`. On the Rails side, you need an initializer of the form:

    ActiveModel::Serializer.setup do |config|
      config.embed = :ids
      config.embed_in_root = true
    end

... and a `key` option in associations to ensure that `key_name` gets serialized as `key_name` instead of `key_name_id`:

    has_one :content, polymorphic: true, key: "content"