Using jest.expect

Hi folks,

I love supertest but I got annoyed by simple assert messages like
```
expected { products: [ { de: [Object], en: [Object] } ] } response body, got { products: [ { de: [Object], en: [Object] } ] }
```

As a quick fix, I often override _assertBody:
```js
const request = require("supertest");

request.Test.prototype._assertBody = (body, res) => {
expect(res.body).toEqual(body);
};
```

This works fine but with 3 restrictions:
- relies on specific assert-implementation e.g. jest
- only works for the body
- only works if request can be match at all

I'd like to see sth like this build into supertest. Before I create a PR, I thought to discuss this. Also, happy for ideas working around mentioned restrictions ;)