Media::encode() causes $data objects that implement JsonSerializer to json_encode incorrectly.

[adallaway]

Media::encode() casts objects in the view data to arrays.
This prevents jsonSerialize() from being called on objects that implement JsonSerializable.

I'm not sure if this is intended behavior but here's a possible fix:
https://github.com/UnionOfRAD/lithium/blob/master/net/http/Media.php#L766

if ($data instanceof \JsonSerializable)) {
  return $data;
}

As a workaround, a dummy to() method can be added to the objects:

public function to()
{
  return $this;
}

[mariuswilms]

Until we find a clean solution for this, try reregistering the json handler with 'cast' set to false. That only works if you always hand JsonSerializable objects to the view.