Update `expand` function for internal convenience.

New strategy for `expand`:

1. if the original resource type is a collection, returns a object keyed
by `data` and `meta`, where `data` stores the collection, and `meta`
stores some info for, say, pagination, etc.
2. if the original resource type is an item, returns the item directly,
no `data` key, no `meta` key, just the item, simply and directly.