Update guides/source/active_record_validations_callbacks.md

In the expiration_date_cannot_be_in_the_past validation method, use `expiration_date.present?` instead of the double negative `!expiration_date.blank?`. Also join the comparisons with `&&` instead of `and`, which could cause unintended consequences.

guides/source/active_record_validations_callbacks.md

@@ -674,7 +674,7 @@ class Invoice < ActiveRecord::Base
     :discount_cannot_be_greater_than_total_value
 
   def expiration_date_cannot_be_in_the_past
-    if !expiration_date.blank? and expiration_date < Date.today
+    if expiration_date.present? && expiration_date < Date.today
       errors.add(:expiration_date, "can't be in the past")
     end
   end