Rework to using classes instead of prototypes

[43081j]

In much of this library, we use functions as constructors and add methods to the prototype, using util.inherits for inheritance

We should probably rework this to use classes now that they're widely available.

For example: https://github.com/chaijs/chai-http/blob/cc58a4b67920b70131969ffa450f53810a2c92dd/lib/request.js#L307-L318

Could be:

class TestAgent extends (Agent || Request) {
  constructor(app) {
    super();
    if (typeof app === 'function') app = http.createServer(app);
    this.app = app;
    if (typeof app !== 'string' && app && app.listen && app.address && !app.address()) {
      this.app = app.listen(0)
    }
  }

  close(callback) {
    // ...
  }
}