ActiveRecord incorrect update query when using custom primary key

gistfile1.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
source 'https://rubygems.org'
gem 'rack', github: 'rack/rack'
gem 'rails', github: 'rails/rails'
gem 'arel', github: 'rails/arel'
gem 'sqlite3'
gem 'i18n', github: 'svenfuchs/i18n'
GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :programs, id: false do |t|
    t.integer :vendor_id
  end
end

class Program < ActiveRecord::Base
  self.primary_key = 'vendor_id'
end

class BugTest < Minitest::Test
  def test_update_primary_key_value
    program = Program.create!(vendor_id: 1)
    program.vendor_id = 2
    # This generates:
    #   UPDATE "programs" SET "vendor_id" = ? WHERE "programs"."vendor_id" = 2  [["vendor_id", 2]]
    program.save!

    program = Program.first
    assert_equal 2, program.vendor_id
  end
end