add arbiter handle assoc fn (#274)

* add arbiter handle assoc fn

actix-rt/tests/tests.rs

@@ -122,6 +122,28 @@ fn arbiter_spawn_fn_runs() {
     arbiter.join().unwrap();
 }
 
+#[test]
+fn arbiter_handle_spawn_fn_runs() {
+    let sys = System::new();
+
+    let (tx, rx) = channel::<u32>();
+
+    let arbiter = Arbiter::new();
+    let handle = arbiter.handle();
+    drop(arbiter);
+
+    handle.spawn_fn(move || {
+        tx.send(42).unwrap();
+        System::current().stop()
+    });
+
+    let num = rx.recv_timeout(Duration::from_secs(2)).unwrap();
+    assert_eq!(num, 42);
+
+    handle.stop();
+    sys.run().unwrap();
+}
+
 #[test]
 fn arbiter_drop_no_panic_fn() {
     let _ = System::new();