supertest fails when using cookie domain / header host

app.js

var koa     = require('koa');
var session = require('koa-session');

var app = module.exports = koa();

app.keys = ['some secret hurr'];
app.use(session({ domain: '.app.localhost' }, app)); // THIS WORKS IN BROWSER BUT FAILS IN SUPERTEST
//app.use(session(app)); // THIS WORKS EITHER WAY

app.use(function*() {
    var n = this.session.views || 0;
    this.session.views = ++n;
    this.body = n + ' views';
    console.log(this.host, n);
});

app.listen(3000);
console.log('listening on port 3000');

package.json

{
  "name": "supertest-session-test",
  "version": "0.0.0",
  "main": "app.js",
  "scripts": {
    "start": "node --harmony app.js",
    "test": "mocha --harmony test.js"
  },
  "dependencies": {
    "koa": "^0.18.0",
    "koa-session": "^3.1.0"
  },
  "devDependencies": {
    "chai": "^2.0.0",
    "mocha": "^2.1.0",
    "supertest": "^0.15.0"
  }
}

test.js

var request = require('supertest');
var expect  = require('chai').expect;
var app     = require('./app.js');

request = request.agent(app.listen());

describe('session test', function() {
    it('gets first view', function(done) {
        request
            .get('/')
            .set({ Host: 'www.app.localhost' })
            .end(function(err, result) {
                expect(result.text).to.equal('1 views');
                done();
            });
    });
    it('gets second view', function(done) {
        request
            .get('/')
            .set({ Host: 'www.app.localhost' })
            .end(function(err, result) {
                expect(result.text).to.equal('2 views');
                done();
            });
    });

});

This fixes it:

var cookieOpt = module.parent ? { } : { domain: '.app.localhost' };
app.use(session(cookieOpt, app));