class Roda::RodaPlugins::Middleware::Forwarder

def call(env)

pass handling of the request to the next middleware.
If this returns a result, return that result directly. Otherwise,
When calling the middleware, first call the current middleware. 
def call(env)
  res = nil
  call_next = catch(:next) do
    env[@mid.opts[:middleware_env_var]] = true
    res = @mid.call(env)
    false
  end
  if call_next
    res = @app.call(env)
    if modified_headers = env.delete('roda.response_headers')
      res[1] = modified_headers.merge(res[1])
    end
  end
  if handle_result = @mid.opts[:middleware_handle_result]
    handle_result.call(env, res)
  end
  res
end