Allows nullable MonitorField to be populated on creation

[FranciscodeMaussion]

Problem

Monitor field does not gets populated on creation since the previous != current evaluates to False. The problem is when when is setted and not respected on creation. Related issues:

• https://github.com/jazzband/django-model-utils/issues/401
• https://github.com/jazzband/django-model-utils/issues/352

Solution

Now 2 checks are done:

• when is None we use previous != current
• when is not None:
• we use previous != current and check if current in when
• (change here) check if is add and current in when

Commandments

• [x] Write PEP8 compliant code.
• [x] Cover it with tests.
• [x] Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• [x] Pay attention to backward compatibility, or if it breaks it, explain why.
• [x] Update documentation (if relevant).