Change ModelFragment attributes on runtime

I have a use case where I need to add new `attr()`'s to a ModelFragment on runtime.

TL;DR; It involves getting metadata from a remote API, and based on that build the model... a.k.a. dynamic attributes

I have tried unregistering and re-registering the model directly from the store:

```
store.unregister('model:any-model');
store.register('model:any-model');, ModelFragment.extend({...})
```

But that only works during *sync* application `instance-initializers` and `init()` methods (in routes, components, whatever).

If I try doing it after an *async* promise is fulfilled, ex. at `beforeModel()`, it simply doesn't work 😢

Any hint would be greatly appreciated!

The issue names ModelFragment, store.unregister/register, synchronous instance-initializers and init(), and asynchronous beforeModel(). Start by reproducing the registration attempt after the promise resolves and tracing those entry points; done should mean runtime-added attr() definitions are available on the model.

索引モデルが issue の本文から書いたものです。