Mutable events can change the event target

[rj00a]

This test fails:

#[test]
fn change_event_target() {
    let mut world = World::new();

    #[derive(Event)]
    struct E(#[event(target)] EntityId);

    #[derive(Component)]
    struct C;

    world.add_system(|mut r: ReceiverMut<E, &C>| {
        r.event.0 = EntityId::NULL;
    });

    world.add_system(|_: Receiver<E, &C>| {});

    let e = world.spawn();
    world.insert(e, C);

    world.send(E(e));
}