Type safety of created document

I'm using ottoman `v2.5.0`

Can I ask what is this [line](https://github.com/couchbaselabs/node-ottoman/blob/master/src/model/document.ts#L42) about?

In my case it messes up with the type safety of the newly created document.

```ts
const doc = new MyModel({ foo: "bar" });
doc.save(); // There is no method `save`
```