Extract polymorphic belongsTo type from <attr>_type

We are serializing polymorphic belongsTo using `<attr>_id` and `<attr>_type` but we weren't extracting using this same pattern.

I've updated UnderscoredSerializer to use a consistent pattern:

Normalize:
```js
  {
    id: "1"
    minion_id: "12",
    minion_type: "EvilMinion"
  }
```

To:
```js
  {
    id: "1"
    minion: { type: "evilMinion", id: "12" }
  }
```

This may not be the correct way forward, so lets have a discussion before merging.

I have kept the existing hasMany polymorphic pattern which expects this format:

```js
  {
    id: "1"
    minions: [
      { id: "12", type: "evil_minion" }
    ]
  }
```

Whether or not this is the *active model serializer default style*, I am uncertain.