Include initializing the supertest Test hash in README examples

[loucadufault]

Previously, the # Usage examples referenced the default exported supertest function of shape (app): SuperagentImplementorFunc (see * Appendix) as if it was the produced function implementing the superagent interface itself (resulting from calling superagent's default export with the initialization app or url).

import request from 'supertest';

// not a valid usage
await request
  .get('/cars') // Becomes /api/cars
  ...

Also, in the example usage with superagent-defaults, the supertest import is unused and the created defaults context does not rely on supertest as illustrated in their docs: https://github.com/camshaft/superagent-defaults#usage .

import supertest from 'supertest';
import defaults from 'superagent-defaults';

...

// Create a defaults context
var request = defaults();

* Appendix

type SuperagentImplementorFunc = {
  [method: string]: (url: string) => superagent.Test
}

in the case of supertest.