Remove invalid resource owner error

Fixes #444.

Changed the `invalid_resource_owner` to `invalid_grant` to comply with the spec.

The second commit (0800715): return the original error description while still returning the correct error code.

It will default to `nil` if not given. If you want to remove them from the doubles in the specs, then we'd need to do something like this in `ErrorResponse`:

```
description_key: (request.respond_to?(:error_description_key) && request.error_description_key)
```

It should probably be better to refactor it so that it builds the error object at the source, from `error` & `error_description_key`.