Rails 中 mattr_accessor 一处文档错误

http://xiewenwei.github.io/blog/2015/01/11/mattr-accessor-in-ruby-on-rails-activesupport/

 module HairColors
   extend ActiveSupport::Concern

   def self.included(b)
     b.class_eval do
       mattr_accessor :hair_colors
     end
     self.class_eval do
       @@hair_color = nil

       def self.hair_colors
         @@hair_color
       end

       def self.hair_colors=(obj)
         @@hair_color = obj
       end
     end
   end
 end

 class Person
   include HairColors
 end

 Person.hair_colors = [:brown, :black, :blonde, :red]
 p Person.hair_colors
 p Person.new.hair_colors
 HairColors.hair_colors=23
 p HairColors.hair_colors

 p Person.hair_colors
 p Person.new.hair_colors