merge upstream update fieldset

lib/active_model/serializer.rb

@@ -21,7 +21,6 @@ module ActiveModel
     def self.attributes(*attrs)
       @_attributes.concat attrs
 
-
       attrs.each do |attr|
         define_method attr do
           object.read_attribute_for_serialization(attr)
@@ -29,6 +28,14 @@ module ActiveModel
       end
     end
 
+    def self.attribute(attr, options = {})
+      key = options.fetch(:key, attr)
+      @_attributes.concat [key]
+      define_method key do
+        object.read_attribute_for_serialization(attr)
+      end unless method_defined?(key)
+    end
+
     # Defines an association in the object should be rendered.
     #
     # The serializer object should implement the association name
@@ -83,8 +90,7 @@ module ActiveModel
     def self.adapter
       adapter_class = case config.adapter
       when Symbol
-        class_name = "ActiveModel::Serializer::Adapter::#{config.adapter.to_s.classify}"
-        class_name.safe_constantize
+        ActiveModel::Serializer::Adapter.adapter_class(config.adapter)
       when Class
         config.adapter
       end