Fix current_item_count counter for DoctrineODMMongoDBAdapt

Hello. For some AdapterInterface of Pagerfanta counter doesn't work correctly. For example  https://github.com/whiteoctober/Pagerfanta/blob/master/src/Pagerfanta/Adapter/DoctrineODMMongoDBAdapter.php. Method ``  count($paginator->getIterator()) ``  doesn't return length current page, because it return length of all results (ignore limit and skip) . Details here: https://github.com/doctrine/mongodb-odm/issues/298.
p.s. I'm sorry for my english