Memoize serialization default directive

Author: bf4

lib/active_model/serializer.rb

@@ -113,6 +113,10 @@ def self.serialization_adapter_instance
       @serialization_adapter_instance ||= ActiveModelSerializers::Adapter::Attributes
     end
 
+    def self.serialization_include_directive
+      @serialization_include_directive ||= { include: '*' }
+    end
+
     attr_accessor :object, :root, :scope
 
     # `scope_name` is set as :current_user by default in the controller.
@@ -166,7 +170,7 @@ def success?
     #     serializer.as_json(include: { posts: { include: { comments: { only: :body } }, only: :title } })
     def serializable_hash(adapter_opts = nil)
       adapter_opts ||= {}
-      adapter_opts = { include: '*' }.merge!(adapter_opts)
+      adapter_opts = ActiveModel::Serializer.serialization_include_directive.merge(adapter_opts)
       serialize(adapter_opts)
     end
     alias to_hash serializable_hash