ID Reberbro can be a string ?

With a string as rederbro ID in opv-import.

Got this : 
```bash
requests.exceptions.HTTPError: {'status': 400, 'errors': [{'validationOf': {'type': ['integer', 'null']}, 'path': ['id_rederbro']}], 'message': 'Bad Request'}
```

A rederbro ID can be an string ? would be better.