Fixes uncaught RuntimeException 'Property "iid" 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
Merge pull request #16 from berni2288/master-fixpropertyerror

Fixes uncaught RuntimeException 'Property "xyz" does not exist