Change Event.Next to send data as Event.Success if last handler

[jpwilliams]

Currently, performing event.Next <- true on the last handler in a chain will treat the message as a success but will not send any data that's been passed, meaning event.Success must be used on the last handler.

WARNING We can't change Event.Next to always send data back as there may well be another handler in the chain. Maybe we could detect this and, if it's the last, return data.