Allow attributes to be "unset"

Allows attributes to be unset, meaning set to `undefined`

This allows the modella-validators plugin to safely ignore values that are semi-temporary

For example, a temporary email field which has `{format: 'email'}` as an option cannot be set to `null` or `false` without failing the validation. When the field is `undefined` however, the validation is ignored.

The modella-mongo plugin is handling this by setting the values to be "null" when they return from an update operation. I will be doing some work to rectify this.