# my User model

  before_validation :update_password

  def update_password
    return unless self.password_changed?
    self.changed_attributes.delete(:password) if self.password.blank?
  end

# Tests

  test "test updating empty password" do
    user = create_user # this method creates a user with random data

    original_id = user.id
    original_password = user.password

    user.password = nil
    assert user.valid?(:update), user.errors.inspect # this is passing

    user.password = nil
    user.save
    saved_user = User.find(original_id)
    assert_equal original_password, saved_user.password # this is line 87
  end

# RESULTS

1) Failure:
test_test_updating_empty_password(UserTest) [test/unit/user_test.rb:87]:
<"$2a$10$rU0at9jcjYRfr5f9NlBMKumSD/NtrCPcAlD5M/edIOgrs5DrLfkXu"> expected but was
<nil>.