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).
`3.1` is end of life, I don't think we need to support it here.