clean the errors if an object that includes validations errors is duped,for 3-2-stable

It Fixes #5953 in 3-2-stable, it's the same pull request of #6284

activemodel/lib/active_model/validations.rb

@@ -165,6 +165,12 @@ module ActiveModel
       end
     end
 
+    # Clean the +Errors+ object if instance is duped
+    def initialize_dup(other) # :nodoc:
+       @errors = nil
+       super
+    end
+
     # Returns the +Errors+ object that holds all information about attribute error messages.
     def errors
       @errors ||= Errors.new(self)

activemodel/test/cases/validations_test.rb

@@ -339,4 +339,19 @@ class ValidationsTest < ActiveModel::TestCase
     end
     assert_equal "Title can't be blank", exception.message
   end
+
+  def test_dup_validity_is_independent
+    Topic.validates_presence_of :title
+    topic = Topic.new("title" => "Litterature")
+    topic.valid?
+
+    duped = topic.dup
+    duped.title = nil
+    assert duped.invalid?
+
+    topic.title = nil
+    duped.title = 'Mathematics'
+    assert topic.invalid?
+    assert duped.valid?
+  end
 end