Shopify/active_merchant — pull request #224 by Nielsomat (2011-11-16 11:39:29+00:00)
Title: Add support for PayPal Express "Digital Goods" / Micropayments

This is essentially #119 rebased onto master with some minor fixes.

From kenmazaika's original description:

```
This type of integration with PayPal allows a more integrated approach, where instead of bouncing the user to PayPal in the same window, then bouncing the user back, it allows the user to add JavaScript to their site to do the redirection inside an iframe/lightbox. This integration method has certain limitations, and differences;

CHANGES:
-different redirection url
-build_setup_request => calling this method in the DG integration requires additional parameters (need to include itemized options with category of all the items 'Digital').
-i had to tweak the PayPal Express base class to support additional fields when setting up a request. These fields are allowed in any request, but required for a DG integration
-Digital Goods Integration is only supported at 65+ Version. Upped the version to 72 since that is the latest
-changed the redirect_urls from being cattr_accessor (using the same class variable) to class_inheritable_accessor (plays nicer with inheritence)
-added test cases for this new gateway.
```