Shopify/active_merchant — pull request #1504 by jcbantuelle (2014-12-30 16:36:11+00:00)
Title: Add FirstData E4 Support for Multiple Mastercard String Formats

The response back from FirstData uses the string 'mastercard'. When using the FirstData Response to reconstruct the credit card info, it needs this key in the BRANDS constant to convert to a valid card.