Fix content-length header for responses with multibyte characters.

Rack::Utils.bytesize seems to be the right choice, as it is also used by Middleman itself: https://github.com/middleman/middleman/blob/8eabe4d354a046f03766521faf42d589801cad36/middleman-core/lib/middleman-more/extensions/minify_css.rb#L56

Fixes “ERR_CONTENT_LENGTH_MISMATCH” error when trying to view the site in Chrome.