Shopify/active_merchant — pull request #875 by christianblais (2013-10-08 19:32:40+00:00)
Title: #checksum_ok? does not crash if tampered

Fixes [Shopify#9594](https://github.com/Shopify/shopify/issues/9594)

During a security challenge, someone forged a request with no invoice, no transaction_status, etc. This ended up in a `NoMethodError`. This patch will prevent that kind of bug to bubble up to Shopify.