Do not serialize nil in serialized attribute.

activerecord/lib/active_record/coders/yaml_column.rb

@@ -15,7 +15,7 @@ module ActiveRecord
       end
 
       def dump(obj)
-        YAML.dump obj
+        YAML.dump(obj) unless obj.nil?
       end
 
       def load(yaml)

activerecord/test/cases/base_test.rb

@@ -1301,11 +1301,24 @@ class BasicsTest < ActiveRecord::TestCase
     assert_equal(myobj, topic.content)
   end
 
-  def test_nil_serialized_attribute_with_class_constraint
+  def test_nil_serialized_attribute_without_class_constraint
     topic = Topic.new
     assert_nil topic.content
   end
 
+  def test_nil_not_serialized_without_class_constraint
+    assert Topic.new(:content => nil).save
+    assert_equal 1, Topic.where(:content => nil).count
+  end
+
+  def test_nil_not_serialized_with_class_constraint
+    Topic.serialize :content, Hash
+    assert Topic.new(:content => nil).save
+    assert_equal 1, Topic.where(:content => nil).count
+  ensure
+    Topic.serialize(:content)
+  end
+
   def test_should_raise_exception_on_serialized_attribute_with_type_mismatch
     myobj = MyObject.new('value1', 'value2')
     topic = Topic.new(:content => myobj)