Workaround for sinatra-assetpack with SSL

app.rb

require 'sinatra'
require 'rack-test-ext'

configure :production do
  use Rack::SSL
  Rack::Test::Session.custom_env['HTTPS'] = 'on'
end

assets do
  css :application, [ '/css/app.css' ]
  prebuild true
end

rack-test-ext.rb

class Rack::Test::Session
  def self.custom_env
    @custom_env ||= {}
  end

  def default_env_with_custom_env
    default_env_without_custom_env.merge(self.class.custom_env)
  end

  alias_method :default_env_without_custom_env, :default_env
  alias_method :default_env, :default_env_with_custom_env
end