Turn on _enableOffering in TryReceiveAll as in TryReceive

In SourceCore TryReceive turns on enableOffering to offer the next messages to any target (if there is one) while TryReceiveAll didn't.

Fix #202
I agree completely about the condition. It was simply copied verbatim from the `TryReceive` method and the guidelines says to keep the original style. They can both be changed however.