class Rack::ContentLength

def call(env)
  status, headers, body = @app.call(env)
  headers = HeaderHash[headers]
  if !STATUS_WITH_NO_ENTITY_BODY.key?(status.to_i) &&
     !headers[CONTENT_LENGTH] &&
     !headers[TRANSFER_ENCODING]
    obody = body
    body, length = [], 0
    obody.each { |part| body << part; length += part.bytesize }
    body = BodyProxy.new(body) do
      obody.close if obody.respond_to?(:close)
    end
    headers[CONTENT_LENGTH] = length.to_s
  end
  [status, headers, body]
end