Make ComposableActor example work. Fixes #2765

Previous implementation called `receive` before any receives were registered. Also `reduce` didn't work on an empty list.

Not 100% sure about making `receive` into a mutable `PartialFunction`. There are probably all sorts of things that can go wrong. But everything should be OK if it's only mutated in `preStart`. There were surely similar problems with the previous implementation too.

I'll make a ticket for forward porting to master. The ticket was originally reported against 2.0. Shall I make ticket for backporting too?