Moving referrer_id to crypt field instead of the form field

In the previous update to [add the referrer_id] (https://github.com/Shopify/active_merchant/commit/82a164c4bce7a360b7d6c75038d46c99137d3db9) it was added as a form field instead of in the crypt field where it should live. 

Added into the correct place and added some tests.

@girasquid @bizla /cc @Shopify/payments