Root node id is serialized as "item_id"

[Danita]

I recently updated my dependency on 2.0.0 alpha to 2.0.0 and I found (the hard way, of course) that the array serialization had changed "item_id" to "id". Not that there is something wrong with that, but it could have been documented.

Anyways I find today that the root node id is still serialized as "item_id", so it's not consistent with the rest of the tree.