Adding type to  when necessary

Adds a type to the `links` key when the relationship name is different than the related object:

```
// Given:
Post = DS.Model.extend({
  comments: DS.hasMany('comment')
});

{
  "id": "1",
    "links": {
      "comments": [ "5", "12", "17", "20" ]
  }
}
```

Now when the name of the realtionship is different to the related type

```
// Given:
Post = DS.Model.extend({
  observations: DS.hasMany('comment')
});

{
  "id": "1",
    "links": {
      "observations": {
        ids: [ "5", "12", "17", "20" ],
       type: "comment"
      }
  }
}
```