Polymorphic belongsTo attributes are camelized by the REST adapter.

This model:

```js
App.Comment = DS.Model.extend({
  blogMessage: DS.belongsTo('message' {
    polymorphic: true
  })
});
```

generates a JSON object like:

```js
{
  "blogMessage": 12,
  "blogMessageType": "post"
}
```
Merge pull request #1325 from Cyril-sf/fix_rest_polymorphic_belongs_to

Polymorphic belongsTo attributes are camelized by the REST adapter.