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).
Can you drop the version number and put this in HEAD, instead? This will likely go in 2.7.0, but we're not sure what version number it will be until we decide to make the release.