make strong SKPSMTPMessageDelegate's instance

[novalagung]

I always got an error while using the latest version of skpsmtpmessage. the error message is [SKPSMTPMessage setWatchdogTimer:]: message sent to de allocated instance. The error is showing when skpsmtp try to invoke implemented delegate.

I try to make SKPSMTPMessageDelegate's instance in SKPSMTPMessage.h to strong and solve this problem.

[brokaw]

This might fix your crash, but it will often result in retain cycles and memory leaks. We can update the delegate to a weak property, which will zero it out and prevent crashes. That will fix the symptom, if not the cause.