about the assert of app.use()
when I run the example.js, just get

```
AssertionError: app.use() requires a generator function
    at Application.app.use (/....../koa/lib/application.js:83:3)
    at Object.<anonymous> (/....../example.js:10:5)
```

I add the `console.log` before assert

``` js
app.use = function(fn){
  console.log(fn.constructor)
  console.log(fn().constructor)
  assert('GeneratorFunction' == fn.constructor.name, 'app.use() requires a generator function');
  debug('use %s', fn.name || '-');
  this.middleware.push(fn);
  return this;
};
```

and the log is:

```
[Function: GeneratorFunction]
[Function: GeneratorFunctionPrototype]
[Function: Function]
[Function: GeneratorFunction]
```
### the most strange is: everything is ok yesterday!
