Currently, aggregate() does the following:

return new \ArrayIterator(array_map(
    function (\stdClass $document) { return (array) $document; },
    $doc["result"]
));

Ideally, we should have a common synthetic Cursor object, which lazily casts stdClass instances into arrays for consistency with the driver's Cursor object. We can later investigate if support type maps (albeit in userland) would be possible.