Force content-type to HTML when it's accepted

I have a case in an ember-cli app where I need to use the `history` location. In order to support it, I want to create copies of `index.html` under public that match my Ember routes. Since Ember routes do not have an extensions, these files cannot end in `.html`, and therefore the broccoli server defaults the mime type to `application/octet-stream` which forces Chrome to download the file.

In lieu of a more configurable broccoli development server, always sending a `Content-Type` of `text/html` when it's accepted worked for me. What do you think?