Serialize Enumerize::Value as string

Currently enumerized field is a `Enumerize::Value` and it is serialized as this object, but it should be `String`. There is no any reason to serialize it as custom object ans save it to database.

`ActiveRecord` uses [YAML.load](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/coders/yaml_column.rb#L13) to serialize object. So the test shows that dumping works as expected.