[core] make actor self reference optional - again

- uses a different method of constructor selection that also works on Apple clang < 8.2.

test/actor/actor.test.cpp

@@ -305,3 +305,34 @@ TEST(Actor, Ask) {
     ASSERT_EQ(std::future_status::ready, status);
     ASSERT_EQ(2, result.get());
 }
+
+TEST(Actor, NoSelfActorRef) {
+    // Not all actors need a reference to self
+    
+    // Trivially constructable
+    struct Trivial {};
+    
+    ThreadPool pool { 2 };
+    Actor<Trivial> trivial(pool);
+    
+    
+    // With arguments
+    struct WithArguments {
+        std::promise<void> promise;
+        
+        WithArguments(std::promise<void> promise_)
+        : promise(std::move(promise_)) {
+        }
+        
+        void receive() {
+            promise.set_value();
+        }
+    };
+    
+    std::promise<void> promise;
+    auto future = promise.get_future();
+    Actor<WithArguments> withArguments(pool, std::move(promise));
+    
+    withArguments.invoke(&WithArguments::receive);
+    future.wait();
+}