Shopify/active_merchant — pull request #656 by duff (2013-04-10 21:24:43+00:00)
Title: PaypalExpress: Remember the response authorization

The Paypal Express #store method uses Paypal's CreateBillingAgreement
API call. The response to this call gives us the billing_agreement_id
which can then be used for future charges via the DoReferenceTransaction
API call.

This commit allows us to consistently ask the response for its
authorization.

https://www.x.com/developers/paypal/documentation-tools/api/createbillingagreement-api-operation-nvp