thoughtbot/shoulda-matchers — pull request #543 by seanpdoyle (2014-06-13 14:49:20+00:00)
Title: Add `validate_uniqueness_of_(:foo).allow_blank`

Complements [`validates_uniqueness_of(:foo, allow_blank:
true)`](http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of).