Includes links inside of linked resources

According to
http://jsonapi.org/format/#document-structure-resource-objects,

> Resource objects have the same internal structure, regardless of
> whether they represent primary or linked resources.

And then
http://jsonapi.org/format/#document-structure-resource-object-attributes,

> There are four reserved keys in resource objects:
>
> "id"
> "type"
> "href"
> "links"

This commit includes `links` inside of linked resources. This way, something like `include: "comment.author"` won't lose the reference (given `comment` does not include `links`).