stream: avoid pause with unpipe in buffered write

If a pipe is cleaned up (due to unpipe) during a write that
returned false, the source stream can get stuck in a paused state.

Fixes: https://github.com/nodejs/node/issues/2323
PR-URL: https://github.com/nodejs/node/pull/2325


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>