Add auto-inclusion of linked and main resources for other resources

Huge proposition. Here the current problem:

- Let’s say we have a `Project` model and a `Map` model.
- And we call `/projects?include=maps`.

Here is the payload:

```json
{
    "projects": [
        {
            "links": {
                "maps": [
                    "8"
                ]
            },
            "id": "7",
            "name": "The Red Rose of Scotland"
        }
    ],
    "linked": {
        "maps": [
            {
                "links": {
                    "project": {
                        "href": "/projects/7",
                        "id": "7",
                        "type": "projects"
                    }
                },
                "id": "8",
                "name": null,
                "custom_map": false,
                "position": null,
                "tiles_directory": null
            }
        ]
    }
}
```

We can see in the `linked.maps[0].links.project` that the lib **can’t detect** that the project is already in the payload... :sadface:

So this pull request does one simple thing:

- It adds `included_models` in the `linked_collection` if it’s available in the model reflection.

What do you think?