(#32476) has_many does not update on save if parent has not been changed

spec.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  gem 'rails', '~> 5.0.0', '>= 5.0.0.1'
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# 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 :authors, force: true do |t|
    t.integer :post_id
    t.string :address, :length => 250
  end

  create_table :posts, force: true do |t|
    t.string :content, :length => 250
  end
  
  create_table :likes, force: true do |t| 
      t.string :like_reason, :length => 250
  end

  create_table :likes_posts, force: true do |t|
    t.integer :post_id
    t.integer :like_id
  end
end

class Author < ActiveRecord::Base
    belongs_to :post
end

class Post < ActiveRecord::Base
  has_and_belongs_to_many :likes
  has_many :authors, :autosave => true
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class Like < ActiveRecord::Base
  has_and_belongs_to_many :posts, :autosave => true 
end

class BugTest < Minitest::Test

  def test_authors_address_must_save_if_there_is_change_in_post
    post = Post.new(content: 'Some text.')
    author = Author.create!(address: 'Mars')
    post.authors << author
    post.save!

    like = Like.new(like_reason: 'Good content.')
    post = Post.find(post.id)
    post.content = 'Some edited content'
    like.posts << post
    author = post.authors[0]
    author.address = 'Earth'
    assert_equal true, author.changed?
    assert_equal true, post.changed?
    like.save!
    
    author.reload
    assert_equal author.address, 'Earth'
  end 

  def test_authors_address_must_save_if_there_is_no_change_in_post
    post = Post.new(content: 'Some text.')
    author = Author.create!(address: 'Mars')
    post.authors << author
    post.save!


    like = Like.new(like_reason: 'Good content.')
    post = Post.find(post.id)
    like.posts << post
    author = post.authors[0]
    author.address = 'Earth'
    assert_equal true, author.changed?
    assert_equal false, post.changed?
    like.save!
    
    author.reload
    assert_equal author.address, 'Earth' 
    
  end
end