make return value of onComplete discardable #61

Hi,

"onSuccess" is non-discardable because most of the time we have to handle "onFail" event. That's great, but I think "onComplete" should be @discardableResult because in most cases, people are gonna handle success, failed and canceled events within one block.

For now, I have to add .ignoreFailure() at the tail which is somewhat redundant. Any good reason that onComplete should not be discardable?

Thanks,
Justin Yan
justin@justinyan.me