Update selfobituary to be used when killed by an unowned actor

When a friend and I were testing this feature, we thought it was a bit of a shame that selfobituary didn't work for unowned actors since we were hoping we could use it for adding special obituaries to stage hazards. This topic just makes the small change to have it apply in that instance as well, since I figure it's a similar enough situation to the original, intended use case.