Add regression test for helper bug
user
```ruby
require 'test_helper'

class HelperTest < Test::Unit::TestCase
  include ActiveMerchant::Billing::Integrations

  def test_mappings_gets_initialized
  	helper_klass_without_mappings = Class.new(ActiveMerchant::Billing::Integrations::Helper)
  	# assert_equal Hash.new, helper_klass_without_mappings.mappings

  	assert_nothing_raised { helper_klass_without_mappings.new(123,'some_key', :amount => 500) }
  end
end
```
assistant