just some indentation fixes
`ObjectModel::add()` does not handle `unset($this->id)` anymore

Unsetting models ID should be done prior to calling this method.
This allows inserting an entity with a previously set ID, which may come in handy when regularely syncing PrestaShop with an external system (an AS400 database for exemple), avoiding in some case the need for an extra column storage to keep the external ID.