New deref from an already deref'd model doesn't include the parent model's path.

[trxcllnt]

@michaelbpaulson @sdesai @jhusain I don't have time to submit a PR right this second, but this line needs to be:

    _path: this._path.concat(nextPath)

This comes up if you do:

root.get('key1.key2.leafValue')
  .flatMap(({json}) => {
    var model1 = root.deref(json.key1);
    return model1.get('key2.leafValue').map(({json} => model1.deref(json.key2))
  })
  .subscribe((model2) => {
    console.log(model2._path); // will be ['key2'] instead of ['key1', 'key2']
  });

Obviously contrived example, but you get the point. Since the second get's JSON is relative to "key1", calling deref with the result of the second get doesn't include key1 like it should.