small bugfix

Its `stream.Stream` where `stream` is the core module : https://github.com/mikeal/request/blob/7430bf38ad032f732cced048570634d8de42cec0/request.js#L7 so : the old style *readable* stream : https://github.com/joyent/node/blob/22c68fdc1dae40f0ed9c71a02f66e5b2c6353691/lib/stream.js#L39-L44