Fixes uncaught RuntimeException 'Property "xyz" does not exist

Fatal error:  Uncaught exception 'Gitlab\Exception\RuntimeException' with message 'Property "iid" does not exist for Gitlab\Model\MergeRequest object' in lib\Gitlab\Model\AbstractModel.php:50

New properties have been added in some Gitlab API models, for example "iid" for merge requests.

This change should make php-gitlab-api more flexible when new properties are added in Gitlab.