jimweirich/builder — pull request #32 by sanemat (2013-01-02 20:48:30+00:00)
Title: Change to String, raise_error last argument accept String

http://apidock.com/ruby/Test/Unit/Assertions/assert_raise
If the last argument is a String, it will be used as the error message.

My environment: Ruby 2.0.0-preview2