Send correct content type for JavaScript files.

[markuspoerschke]

The JavaScript files should be send with the correct content type header. Otherwise the browser might show a warning in th console:

[electerious]

The tests are failing. It seems that setHeader is not a function in that case:

TypeError: Cannot read property 'setHeader' of undefined 

[electerious]

The routes will be pre-executed when in production mode. req and res don't exist in this case. I might need to rewrite the preload function to allow custom headers. This is also necessary to serve a favicon with the correct content-type: #38

[electerious]

Implemented: 3f11e09