Fix ActiveModel::Errors#dup

Since ActiveModel::Errors instance keeps all error messages as hash
we should duplicate this object as well.

Previously ActiveModel::Errors was a subclass of ActiveSupport::OrderedHash,
which results in different behavior on dup, this may result in regression for
people relying on it.

Because Rails 3.2 stills supports Ruby 1.8.7 in order to properly fix this
regression we need to backport #initialize_dup.

activemodel/test/cases/errors_test.rb

@@ -46,6 +46,14 @@ class ErrorsTest < ActiveModel::TestCase
     assert errors.has_key?(:foo), 'errors should have key :foo'
   end
 
+  def test_dup
+    errors = ActiveModel::Errors.new(self)
+    errors[:foo] = 'bar'
+    errors_dup = errors.dup
+    errors_dup[:bar] = 'omg'
+    assert_not_same errors_dup.messages, errors.messages
+  end
+
   test "should return true if no errors" do
     person = Person.new
     person.errors[:foo]