YamlSourceManipulator adding item to the array after array leads to bug

[pincher2012]

Example of config:

framework:
    messenger:
        routing:
            App\Messages\OldMessage: [ transport1, transport2 ]
===
$data['framework']['messenger']['routing']['App\Messages\NewMessage'] = 'async';
===
framework:
    messenger:
        routing:
            App\Messages\OldMessage: [ transport1, transport2 ]
            App\Messages\NewMessage: async

PR with failed test: #1665