When calling resource.notify('subj', 'body') I believe the sender should be set. Why was this not implemented? Before I do a pull request I want to make sure I'm not missing something.
A notification is a message from the system to a Messageable or group of Messageables. Therefore, there is no sender that your application should be worried about since the sender is the application. If you are looking to create a conversation between two people, look at Mailboxer::Models::Messageable#send_message