Shopify/active_merchant — pull request #1495 by ntalbott (2014-12-18 21:28:51+00:00)
Title: PayPal: Allow specifying ButtonSource at init

In some environments one may want to set the ButtonSource on a per-
gateway basis; this allows overriding it in the @options has by
passing in a :button_source option.

If no :button_source is provided, application_id continues to be used
if available.

Please review @duff @girasquid.