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:

chai-http/lib/request.js

Lines 307 to 318 in cc58a4b

	function TestAgent(app) {
	if (!(this instanceof TestAgent)) return new TestAgent(app);
	if (typeof app === 'function') app = http.createServer(app);
	(Agent || Request).call(this);
	this.app = app;
	if (typeof app !== 'string' && app && app.listen && app.address && !app.address()) {
	this.app = app.listen(0)
	}
	}
	util.inherits(TestAgent, Agent || Request);
	TestAgent.prototype.close = function close(callback) {

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) {
    // ...
  }
}