nil in a serialized column should serialize to NULL, as it did in all…

… previous versions of Rails

activerecord/lib/active_record/coders/yaml_column.rb

@@ -15,7 +15,7 @@ def initialize(object_class = Object)
       end
 
       def dump(obj)
-        YAML.dump obj
+        YAML.dump obj unless obj.nil?
       end
 
       def load(yaml)

activerecord/test/cases/base_test.rb

@@ -1222,6 +1222,13 @@ def test_serialized_string_attribute
     assert_equal(myobj, topic.content)
   end
 
+  def test_serialized_nil_attribute_is_null_in_database
+    myobj = nil
+    topic = Topic.create("content" => myobj).reload
+    assert_equal(myobj, topic.content)
+    assert Topic.where("content IS NULL").exists?(topic.id)
+  end
+
   def test_nil_serialized_attribute_with_class_constraint
     topic = Topic.new
     assert_nil topic.content

activerecord/test/cases/coders/yaml_column_test.rb

@@ -19,6 +19,7 @@ def test_type_mismatch_on_different_classes
       def test_nil_is_ok
         coder = YAMLColumn.new
         assert_nil coder.load "--- "
+        assert_nil coder.load nil
       end
 
       def test_returns_new_with_different_class