[9.x] Allows object instead of array when adding to PendingBatch

[yoeriboven]

Currently you need to pass an array when you want to add to a batch.

$batch = Bus::batch([]);
$batch->add([new Job]);
$batch->dispatch();

With this PR you can do this:

$batch = Bus::batch([]);
$batch->add(new Job);
$batch->dispatch();

This is already possible if you want to add to an already dispatched batch:

$this->batch()->add(new Job);

The add function on PendingBatch wasn't tested yet either so I added a test.