Rails 4.2 cannot serialize certain strings when PostgreSQL is used

pg_bytea_serialize.rb

require 'yaml'

gem 'activerecord', '4.2.0'
require 'active_record'
require 'minitest/autorun'
require 'logger'

Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: '...', username: '...', password: '...')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  # drop_table :posts rescue nil
  create_table :posts do |t|
    t.binary :attachment
  end
end

class Post < ActiveRecord::Base
  serialize :attachment
end

class BugTest < Minitest::Test
  def test_serialize
    Post.create!(attachment: '\/')
  end
end