Fix refList bugs with nested paths

See https://groups.google.com/forum/?fromgroups=#!topic/derbyjs/oLr2xlFjGcQ

Test case:

```js
model.set("_data", {
	21: { id: 21, name: 'a' },
	31: { id: 31, name: 'b' },
	41: { id: 41, name: 'c' }
});
model.ref("_all", model.filter('_data'));
model.ref("_subset", model.filter("_all").where('name').within(['b', 'c']));
 
console.log("get('_subset')[0]:", model.get('_subset')[0]);
console.log("get('_subset.0'):", model.get('_subset.0'));
 
model.set("_keys", [31,41]);
 
model.refList("_items", "_all", "_keys");
 
console.log("get('_items')[0]:", model.get('_items')[0]);
console.log("get('_items.0'):", model.get('_items.0'));
```

Output:

```
get('_subset')[0]: Object {id: 31, name: "b"}
get('_subset.0'): Object {id: 21, name: "a"}
get('_items')[0]: Object {id: 31, name: "b"}
get('_items.0'): undefined 
```

`prevRests` should probably be removed entirely