koa session does not work when used in a middleware app
I am using the vhost middleware with koa-session. Since koa-session 2.0.0 is was possible to use it in a subapp. Since 3.0.0 this does not work anymore. Example:

``` js
var session = require('./');
var koa = require('koa');
var compose = require('koa-compose');
var app = koa();

app.keys = ['some secret hurr'];

app.use(session(app));

app.use(function* (next){
  if ('/favicon.ico' == this.path) return;
  var n = this.session.views || 0;
  this.session.views = ++n;
  this.body = n + ' views';
});

var server = koa();
server.use(compose(app.middleware));
server.listen(3000);

console.log('listening on port 3000');
```
