Akka.Actor.Serialization: remember to serialize envelope AND underlying type when using serialize-messages=on

[Aaronontheweb]

Changes

Bug: need to serialize both the envelope and the underlying type when serialize-messages=on

We found an issue stemming from a change I made in by #7010 where the underlying IWrappedMessage envelope would get discarded when serialize-messages=on. The solution to this problem is to force serialization of the envelope AND the underlying type and return the product of both of those. Right now we are discarding the envelope by accident, which causes the DistributedPubSubMediator to be unable to process any messages when serialize-messages=on

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.

[Aaronontheweb]

Now that #7202 has been merged in, we should be good to go