Update test_serializable_with_model_when_None
Add test to hide serializable when None
Merge pull request #3 from nKey/serializable-when-none

Update Serializable to accept serialize_when_none option
Merge remote-tracking branch 'jokull/master' into jokull

* jokull/master:
  Add test to hide serializable when None
  Update test_serializable_with_model_when_None