Clean up meta handling.

lib/active_model/serializer/meta.rb

@@ -0,0 +1,29 @@
+module ActiveModel
+  class Serializer
+    module Meta
+      extend ActiveSupport::Concern
+
+      included do
+        with_options instance_writer: false, instance_reader: true do |serializer|
+          serializer.class_attribute :_meta # @api private
+        end
+
+        extend ActiveSupport::Autoload
+      end
+
+      module ClassMethods
+        # Set the JSON API meta attribute of a serializer.
+        # @example
+        #   class AdminAuthorSerializer < ActiveModel::Serializer
+        #     meta { stuff: 'value' }
+        # @example
+        #     meta do
+        #       { comment_count: object.comments.count }
+        #     end
+        def meta(value = nil, &block)
+          self._meta = block || value
+        end
+      end
+    end
+  end
+end