avoid nil.dup

Signed-off-by: Santiago Pastorino <santiago@wyeworks.com>

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -41,7 +41,7 @@ module ActiveRecord
             if create_time_zone_conversion_attribute?(attr_name, columns_hash[attr_name])
               method_body, line = <<-EOV, __LINE__ + 1
                 def #{attr_name}=(original_time)
-                  time = original_time.dup
+                  time = original_time.dup unless original_time.nil?
                   unless time.acts_like?(:time)
                     time = time.is_a?(String) ? Time.zone.parse(time) : time.to_time rescue time
                   end

activerecord/test/cases/attribute_methods_test.rb

@@ -460,6 +460,14 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     end
   end
 
+  def test_write_nil_to_time_attributes
+    in_time_zone "Pacific Time (US & Canada)" do
+      record = @target.new
+      record.written_on = nil
+      assert_nil record.written_on
+    end
+  end
+
   def test_time_attributes_are_retrieved_in_current_time_zone
     in_time_zone "Pacific Time (US & Canada)" do
       utc_time = Time.utc(2008, 1, 1)