changeset.set('nested.path', undefined) does not update property

**Version**: 4.1.2

Example:
```
changeset.set('nested.path', true); 
changeset.get('nested.path') // true

changeset.set('nested.path', undefined);
changeset.get('nested.path') // true
```

Possibly related to: https://github.com/adopted-ember-addons/ember-changeset/pull/191

