force to use flattenJson when dealing with ArraySerializer

Author: joaomdmoura

lib/active_model/serializer/adapter/json.rb

@@ -6,7 +6,7 @@ class Adapter
       class Json < Adapter
         def serializable_hash(options = {})
           if serializer.respond_to?(:each)
-            @result = serializer.map{|s| self.class.new(s).serializable_hash }
+            @result = serializer.map{|s| FlattenJson.new(s).serializable_hash }
           else
             @hash = {}