Prevent throwing in .done if callback supplied.

Makes it easier to catch errors.

Idea is turn [this](https://github.com/mmalecki/hock/blob/24fc5e2d98efff89edb6e28ed348843f17c399b8/test/many-test.js#L49-L51):

```js
(function() {
  server.done();
}).should.throw();
done();
```

into:

```js
done(server.done);
```