Fix async relationships

This fixes cases where you would have a async relationship declared like this:

```json
{
  "posts": [
    {
      "id": "1",
      "title": "Foo",
      "links": {
        "comments": {
          "href": "/comments?post_id=1",
          "type": "comments"
        }
      }
    }
  ]
}
```