rspec/rspec-expectations — pull request #534 by jcarbo (2014-04-18 20:08:43+00:00)
Title: Pass original message to custom failure message

When you set the custom failure message of a matcher to a Proc, the
original/default failure message is now passed to the Proc as
block argument.

The impetus for this was wanting to add more detail to a failed test's output
without completely ditching the nicely formatted default failure message.