fix nested property values

i would write a test but i'm honestly stuck with how to get the tests to run.  in lieu of tests, here's a snippet that shows the issue i'm trying to fix with this PR

```js
m = new Model({ object: { foo: 'bar'  }  });
p = m.property('object', 'foo');
console.log(p._get()); // should be 'bar'
```