Question: Use legacy middleware inside of modern middleware
Hello! I've been trying to get this small example to work to try and understand Koa/middleware a bit better, but I ain't getting it to work.

```
'use strict';
const Koa = require('koa');
const app = new Koa();

const convert = require('koa-convert');
const bind = require('co-bind');

function* myGen(next) {
    // before
    console.log('myGen before');
    yield* next;
    // after
    console.log('myGen after');
}

function myFn(ctx, next) {
    let newMyGen = bind(myGen, ctx, next);
    let conMyGen = convert(newMyGen);
    conMyGen();

    // before
    console.log('myFn before');
    return next().then(() => {
        // after
        console.log('myFn after');
    });
}

app.use(myFn);

app.use(function(ctx, next) {
    console.log('body');
    ctx.body = 'done';
});

app.listen(3000);
```

The console displays the following:

```
myGen before
myFn before
body
myFn after
```

I'm not doing something right to get back to the rest of myGen function.
