use $unset in update queries when the attribute has been unset on the model

Use the $unset update modifier when encountering properties that have been unset on the model

The unit tests for this will fail unless you are using the paulbdavis/modella#unset version of the modella module [PR for the unset feature](https://github.com/modella/modella/pull/41)

EDIT: Seems I was wrong  about the tests not passing