[5.6] Add tests for the way JsonResource handles numeric keys

[SjorsO]

JsonResource seems to strip all array keys if any of them are numeric or a numeric string. I couldn't find any tests that assert this behavior, so this PR adds some.

Because of this behavior, doing this will give you an array of names, without any ids:

public function index()
{
    $data = User::pluck('name', 'id');

    return JsonResource::make($data);
}

I'm not sure if this is a bug or a feature, but having tests for behavior like this is important.

[deleugpn]

If there was no test for this before, perhaps we can try to fix it and target 5.7 instead of define it as the desired behavior?

[SjorsO]

These tests are defining it as the current behavior, they help prevent any unintended backward compatibility breaks.

I think it would be best if JsonResource matches the way json_encode() strips keys.

[taylorotwell]

Send to 5.7.