Update asset_path with fonts type for Sprockets helpers

In regard to [Issue #913](https://github.com/middleman/middleman/issues/913), this should fix the wrong font path output for apps using Middleman Sprockets and a custom `:fonts_dir` set in `config.rb`.

I had to add a server step to get sprockets loading in the test environment as well in commit [3b1efdd](https://github.com/jordanandree/middleman/commit/3b1efdd90185e6b3d9b4c966328246fe60efeb86). Hope that isn't a problem.

@mrbinky3000 Would love to know if this branch gets things working for you.