document hash is an array

P.S. Why $id is build with 2 dashes if in DB is seems with one?

```$item_id = $doc_fields_hash . '--' . $item_id;```

seems like that was solved in ongoing 4.x already, but that is only for D10.x

4.2 is not supported anymore and there will be no further release. But thanks for the patch people could apply locally!