Use a Set for managing uniqueness behavior

lib/fast_jsonapi/object_serializer.rb

@@ -80,7 +80,7 @@ module FastJsonapi
 
       return if options.blank?
 
-      @known_included_objects = {}
+      @known_included_objects = Set.new
       @meta = options[:meta]
       @links = options[:links]
       @is_collection = options[:is_collection]

lib/fast_jsonapi/serialization_core.rb

@@ -133,9 +133,9 @@ module FastJsonapi
             end
 
             code = "#{record_type}_#{serializer.id_from_record(inc_obj, params)}"
-            next if known_included_objects.key?(code)
+            next if known_included_objects.include?(code)
 
-            known_included_objects[code] = inc_obj
+            known_included_objects << code
 
             included_records << serializer.record_hash(inc_obj, fieldsets[record_type], includes_list, params)
           end