Always return a messaging service in the PackEncryptedMetadata

[dbluhm]

As implemented, calling pack_encrypted with the to DID not requiring forwarding will cause the returned PackEncryptedMetadata to omit the messaging_service parameter.

I suggest always returning the messaging_service parameter, even when forwarding is not required because not returning it requires me to rewrite routines already implemented by this library to determine the service endpoint to deliver my message to. It's not terribly complicated but it creates an opportunity for subtle discrepancies in implementation resulting in potentially inconsistent behavior.