class Rack::Chunked

def call(env)

modify the response to use chunked transfer-encoding.
but does not have content-length or transfer-encoding headers,
If the rack app returns a response that should have a body, 
def call(env)
  status, headers, body = response = @app.call(env)
  if chunkable_version?(env[SERVER_PROTOCOL]) &&
     !STATUS_WITH_NO_ENTITY_BODY.key?(status.to_i) &&
     !headers[CONTENT_LENGTH] &&
     !headers[TRANSFER_ENCODING]
    headers[TRANSFER_ENCODING] = 'chunked'
    if headers['trailer']
      response[2] = TrailerBody.new(body)
    else
      response[2] = Body.new(body)
    end
  end
  response
end