Empty index results should return an empty array

Given there are no posts in the database:

```json
GET /posts

Currently:
{}

Expected result:
{
  "posts": []
}
```

JSON API does not specifically requires this, but an empty array seems much more reasonable (also existing clients like [ember-json-api](https://github.com/kurko/ember-json-api) expect an array response).

At the moment this is just a failing test, as I'm not sure how to fix the issue.
The problem is that [here](https://github.com/cerebris/jsonapi-resources/blob/master/lib/jsonapi/resource_serializer.rb#L14) there is no way for the serializer to know the current resource type.
One possible solution - passing some kind of ResourceCollection  to the serializer instead of just plain array..