Won't render if options are passed in
I'm trying to set up a single jade file for both SPA's of my app and want to let it load a certain javascript file for each SPA (i.e. load the shop script on the shop SPA and the dashboard script on the dashboard app). I assumed I could have a script tag like this in the single jade file: `script(src=#{js})` and then pass in the correct script like so:

``` javascript
const router = require('koa-router')();

router.get('/', function* (next) {
  yield this.render('index', {
    js: 'javascripts/shop.min.js'
  });
});

module.exports = router;
```

However, when I try to load the shop route, I get a 500 error code in the console and nothing loaded on the page.
