[5.5] Pass test value to Collection::when callbacks

[alepeino]

This brings the method in line with Illuminate\Database\Concerns\BuildsQueries::when, and avoids the need for a use statement when we want the truthy value inside the callback.

For instance:

$item = $items->next();
$collection->when($item, function ($collection) use ($item) {
    return $collection->push($item);
});

can become

$collection->when($items->next(), function ($collection, $item) {
    return $collection->push($item);
});