Probably copying from MediaCollection to singleFile MediaCollection works weird about laravel-medialibrary

[patinthehat]

@drnkwtr I'd be happy to take a look at a PR with failing unit tests that show the issue, as all of the tests are currently passing. Thanks! 👍

[drnkwtr]

    public function selectThumbnail($mod_id, $file_id)
    {
        $media = Media::find($file_id)->copy(Modification::find($mod_id),'thumbnail');
        $media->order_column = 1;
        $media->save();
        return back();
    }

This is the fix of this issue. I think, new media should be copied with order_column depends on count of collection medias.