Do not look for a child root node in relationships

For `belongs_to` and `has_one` relationships, it’s uncommon to see this:

  {
    "user": {
      "id": 1,
      "name": "Rémi",
      "organization": {
        "organization": {
          "name": "Foo Inc."
        }
      }
    }
  }

Instead, we’re more likely to see this:

  {
    "user": {
      "id": 1,
      "name": "Rémi",
      "organization": {
        "name": "Foo Inc."
      }
    }
  }

So even if the Organization model defined `parse_root_in_json true`, we
won’t look for an extra child root node.