Add Doctrine\ODM\PHPCR\ChildrenCollection to ArrayCollectionHandler

Using PHPCR ODM I ran into an issue where it tries to serialize [PHPCRs ChildrenCollection](https://github.com/doctrine/phpcr-odm/blob/master/lib/Doctrine/ODM/PHPCR/ChildrenCollection.php).

@schmittjoh: Should I multiline the array?