Introduced payment_routine method for better service_url handling

[AlexanderZaytsev]

2checkout supports two service urls (see the source link at the end):

Multi-page Payment Routine: https://www.2checkout.com/checkout/purchase

Single Page Payment Routine: https://www.2checkout.com/checkout/spurchase

I added the payment_routine method to be able to switch between these service urls.

def test_single_page_payment_routine_service_url
  TwoCheckout.payment_routine = :single_page
  assert_equal 'https://www.2checkout.com/checkout/spurchase', TwoCheckout.service_url
end

def test_multi_page_payment_routine_service_url
  TwoCheckout.payment_routine = :multi_page
  assert_equal 'https://www.2checkout.com/checkout/purchase', TwoCheckout.service_url
end

I also modified service_url= for backward compatibility so that similar code would keep working (in case someone was doing this):

ActiveMerchant::Billing::Integrations::TwoCheckout.service_url = "https://www.2checkout.com/checkout/spurchase"

Source: https://www.2checkout.com/blog/knowledge-base/merchants/tech-support/3rd-party-carts/parameter-sets/pass-through-product-parameter-set/