create test for checking with_id

It seems like the entity itself is changing when
adding a component.

Not sure if it's a bug in with_id() or something deeper.

tests/lib.rs

@@ -1738,6 +1738,55 @@ fn not_unique_storage() {
     }
 }
 
+#[test]
+fn key_equality() {
+    let world = World::default();
+    world.register::<usize>();
+    world.register::<u32>();
+
+    //create 3 entities
+    let (e0, e1, e2) = world.run::<(EntitiesMut, &mut usize), _, _>(
+        |(mut entities, mut usizes)| {
+            (
+                entities.add_entity(&mut usizes, 0), 
+                entities.add_entity(&mut usizes, 1), 
+                entities.add_entity(&mut usizes, 2)
+            )
+        },
+    );
+
+    //add a component to e1 
+    world.run::<(EntitiesMut, &mut u32), _, _>(
+        |(ref mut entities, ref mut u32s)| {
+            entities.add_component(u32s, 42, e1);
+        }
+    );
+
+    //confirm that the entity keys have not changed for usizes storage
+    world.run::<&usize, _, _>(
+        |usizes| {
+            //sanity check
+            assert_eq!((&usizes).iter().with_id().count(), 3);
+
+            let keys:Vec<Key> = (&usizes).iter().with_id().map(|(entity, _)| entity).collect();
+
+            assert_eq!(keys, vec![e0, e1, e2]);
+        }
+    );
+
+    //confirm that the entity key for (usize) is the same as (usize, u32) 
+    //in other words that the entity itself did not somehow change from adding a component 
+    world.run::<(&usize, &u32), _, _>(
+        |(usizes, u32s)| {
+            //sanity check
+            assert_eq!((&usizes, &u32s).iter().with_id().count(), 1);
+
+            let (entity, _, _) = (&usizes, &u32s).iter().with_id().next().unwrap();
+            assert_eq!(entity, e1);
+        }
+    );
+}
+
 #[test]
 fn unique_storage_pack() {
     let world = World::new::<(u32,)>();