included_spec.rb

require 'spec_helper'

class Test
  def self.messages
    @messages ||= []
  end
end
describe Yahoo do
  let(:yahoo_included_test_object) do
    Test.send(:include, Yahoo)
    Test.new
  end
  let(:test_object) do
    Test.new
  end
  it { yahoo_included_test_object.class.messages.should include(:yahoo)} #여기까지는 괜찮음
  it { test_object.class.messages.should_not include(:yahoo)} # 실패 함!
end

yahoo.rb

module Yahoo
  self.included(base)
    base.messages << :yahoo
  end
end