Fix ChildrenCollection takeSnapshot bug

The ``takeSnapshot`` method of the ``ChildrenCollection`` appends the keys of the collection creating a nested array. If you flush twice this will result in an ``Array to string conversion`` error in  [UnitOfWork.php#L1180](https://github.com/doctrine/phpcr-odm/blob/f21d35eb7f4a169f80e12b76c708d644a93158d5/lib/Doctrine/ODM/PHPCR/UnitOfWork.php#L1180).