Has_secure_password `validate :password, if: :present?` not working on an existing record

has_secure_password_password_validation_bug_test.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.1.1'
require 'sqlite3'
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 :users do |t|
    t.string :password_digest
  end
end

class User < ActiveRecord::Base
  has_secure_password

  validates :password, length: { minimum: 8 }, if: :present?
end

class HasSecurePasswordPasswordValidationBugTest < Minitest::Test
  def test_has_secure_password_validates_if_present_correctly
    user = User.create!(password: "password")

    assert user.valid? # passes
    assert user.reload.valid? # passes

    user = User.last
    assert user.valid? # fails
  end
end