application.rb

# Configure Rack middleware
config.middleware.insert_before 0, 'WwwRedirect'

www_redirect.rb

class WwwRedirect
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new env
    if request.host.starts_with? 'www.'
      [301, { 'Location' => request.url.sub('//www.', '//'), 'Content-Type' => 'text/html' }, self]
    else
      @app.call env
    end
  end

  def each(&block)
  end
end