GET /conversations/open returns _id instead of id

[Francois Falala-Sechet]

Describe the bug
The return of this endpoint is incorrect:

{
  "_id": "555416aa-c033-4676-ad64-eaf5c6783617", <= should be id, not _id
  "client": { ... },
  "flow_id": "Default",
  "step_id": "start",
  "metadata": { ... },
  "status": "OPEN",
  "last_interaction_at": "2021-01-22T20:58:39.752Z",
  "updated_at": "2021-01-22T20:58:39.664Z",
  "created_at": "2021-01-22T20:58:39.664Z"
}