Mongo insert bug with the magic add a _id even if the data is passed by copy

gistfile1.php

<?php

class EntityObject {
    private $arData = array();

    public function __construct ($initialValue) {
        $this->setProperty($initialValue);
    }

    public function setProperty ($value) {
        $this->arData['property'] = $value;
    }

    public function getProperties () {
        return $this->arData;
    }
}
$entity = new EntityObject('foo');

$mongoClient = new MongoClient();
$toInsert = $entity->getProperties();
$mongoClient->fooDb->fooCollection->insert($toInsert, array('fsync'=>true));

echo "toInsert should be updated with _id \n";
print_r($toInsert);

echo "private data should not be updated with _id \n";
print_r($entity->getProperties());

/*
Expected result : 
toInsert should be updated with _id 
Array
(
    [property] => foo
    [_id] => MongoId Object
        (
            [$id] => 52090a28945630cf261e34fc
        )
)
private data should not be updated with _id 
Array
(
    [property] => foo
)

Actual result
toInsert should be updated with _id 
Array
(
    [property] => foo
    [_id] => MongoId Object
        (
            [$id] => 52090a28945630cf261e34fc
        )

)
private data should not be updated with _id 
Array
(
    [property] => foo
    [_id] => MongoId Object
        (
            [$id] => 52090a28945630cf261e34fc
        )

)