Merge pull request rails#6329 from acapilleri/backport_dup_validation_3_2

Backport dup validation 3 2

Author: rafaelfranca

activemodel/lib/active_model/validations.rb

@@ -168,7 +168,15 @@ def inherited(base)
     # Clean the +Errors+ object if instance is duped
     def initialize_dup(other) # :nodoc:
       @errors = nil
-      super
+    end
+    
+    # Backport dup from 1.9 so that #initialize_dup gets called
+    unless Object.respond_to?(:initialize_dup)
+      def dup # :nodoc:
+        copy = super
+        copy.initialize_dup(self)
+        copy
+      end
     end
 
     # Returns the +Errors+ object that holds all information about attribute error messages.