Pass fields down from constructor

lib/active_model_serializers/adapter/attributes.rb

@@ -3,6 +3,7 @@ module ActiveModelSerializers
     class Attributes < Base
       def serializable_hash(options = nil)
         options = serialization_options(options)
+        options[:fields] ||= instance_options[:fields]
         serializer.serializable_hash(instance_options, options, self)
       end
     end