Unshunting a shunted message can skip the original shunting handler.

[Mark Sapiro]

At least sometimes, when a message is shunted because an exception is thrown in IncomingRunner the pipeline saved in the msgdata of the shunted message can start with the handler following the one being handled when the exception is thrown.

This has been observed when Moderate attempted to reject a message and raised Errors.RejectMessage. IncomingRunner then called BounceMessage to send a reject notice and BounceMessage threw an IOError exception. The message was shunted with a pipeline starting with the handler following Moderate which had already been successfully popped from the pipeline. When the message was unshunted, Moderate was skipped and the message which should have been rejected was delivered to the list.