Regression from 3.2.18 to 4.0.5

issue_13747.rb

# Activate the gem you are reporting the issue against.
# gem 'activerecord', '3.2.18'
gem 'activerecord', '4.0.5'
require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
    t.string :type, null: false
    t.string :content, null: false
  end
end

class Post < ActiveRecord::Base
end

class VideoPost < Post
end

class ImagePost < Post
end

class BugTest < Minitest::Test
  def test_becomes_after_dup_saves_all_attributes
    video = VideoPost.create!(content: 'foo')

    image = video.dup.becomes(ImagePost)
    image.save!

    assert_equal 'foo', image.content
  end
end