console.log(require.cache) fails when koa is present
``` js
koa = require('koa')
console.log(require.cache)
```

will fail because of the custom `inspect` implementation in [`context.js`](https://github.com/koajs/koa/blob/master/lib/context.js#L27), which uses the custom `toJSON`, which relies on a properly set `this`, which is not the case.

not very important. But noteworthy - I guess
At least is costed me some time to figure it out.
