Uniqueness validation bug in Rails 4.1.10.rc1

gistfile1.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', '4.1.10.rc1'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 :posts, force: true  do |t|
    t.string :title
    t.integer :author_id
  end
end

class Post < ActiveRecord::Base
  self.table_name = :posts

  validates :title, uniqueness: { scope: :author_id }
end

class BugTest < Minitest::Test
  def test_uniqueness_with_scope_and_value
    post = Post.new(title: 'test_1', author_id: 42)
    assert post.save, "Should save post as unique"

    post_2 = Post.new(title: 'test_1', author_id: 42)
    assert !post_2.valid?, "Shouldn't be valid"
    assert !post_2.save, "Shouldn't save post_2 as unique"
  end

  def test_uniqueness_with_scope_and_nil
    post = Post.new(title: 'test_2', author_id: nil)
    assert post.save, "Should save post as unique"

    post_2 = Post.new(title: 'test_2', author_id: nil)
    assert !post_2.valid?, "Shouldn't be valid"
    assert !post_2.save, "Shouldn't save post_2 as unique"
  end
end