Support re-run activity that using sagaPropertBag to pass information

[majdee]

We're using SagaPropertBag to pass parameters between different activities, and we have a re-try mechanism to re-run failing activities, but in case we run the same activity that already added a key to the bag, we will get an exception that key already exists.

This fix should fix the issue above.

[creyke]

Thanks for the pull request. I notice it triggered a NuGet publish build which I'm looking into as this shouldn't happen until it is merged.