Set headers with Sinatra helpers

Less code! :smile: 

According to [docs](http://rubydoc.info/gems/sinatra/Sinatra/Helpers#expires-instance_method), `expires` sets both `Cache-Control` and `Expires`, taking just an integer argument.

[`content_type`](http://rubydoc.info/gems/sinatra/Sinatra/Helpers#content_type-instance_method) is a little cleaner too.