incorrect headers on CSS style sheet

[acarl005]

This is a really cool project! There is one thing I found that could be improved upon. I tried adding the following URL to a <style> tag.

https://npm-cdn.herokuapp.com/graphsiql@0.0.4/graphiql.css

And I got the following warning in my Chrome console:

Resource interpreted as Stylesheet but transferred with MIME type text/plain: "https://npm-cdn.herokuapp.com/graphsiql@0.0.4/graphiql.css".

It would be nice if it set the correct Content-type header similar to how Raw Git does