Polymorphic fragmentArray and typeKey

Hi

I used to be able to do this on a polymorphic model to get to the typekey given as argument in the model definition

```
const typeKey = this.contactModel.get('options.typeKey');
```

Currently (6.0.1) these event options is undefined. Did ember data remove that? I'm still using ember and ember-data 3.28.12

```
@fragmentArray('fragment/contact/contactcomponent', {
polymorphic: true,
typeKey: 'componentType',
...
```

The issue names fragmentArray with polymorphic: true and typeKey: 'componentType', but no source file or test. Start by reproducing the missing options.typeKey behavior in the reported Ember Data 3.28.12 and 6.0.1 setups, then trace fragmentArray's option handling. Done means the typeKey is available as expected, with a regression test covering the polymorphic model.

索引モデルが issue の本文から書いたものです。