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

[SjorsO]

(#25226 retargeted to 5.7)

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.

[staudenmeir]

I and others would call this a bug that should be fixed (#24339, #23595, #24302). At least, the unused removeMissingValues() parameter should be fixed.

[taylorotwell]

It's not a bug.

[deleugpn]

I can see how this would be treated as not-a-bug because the Resource API layer is only interested in giving back a collection of objects or a single object and in either case the key is no longer in the Eloquent's power as it's a whole other software layer. I usually include 'id' => $this->resource->id in my Resource Objects, allowing frontend to pluck the same way we do with Collections. I guess I'm neutral on this one as I can see it working both ways.

[staudenmeir]

I mainly consider it a bug because #23414 unintentionally changed the behavior in a minor release.

You can argue about whether numeric keys make sense in JSON responses. But by allowing them, you at least give people a choice. You don't have to use them.