Fix serializing when path has integer bits

[khepin]

Deserializing #[Json('list', 0, 'name')] was already fine but the issue was when re-serializing this.

Using stdClass meant it was impossible to set something at the property 0. By switching internally to an array, we can add properties and / or integer offsets correctly.

@mattgrande @bezhermoso @szainmehdi