Set the `req.buffer` property, which serves as a way of keeping
a `Buffer` alive that is being written to a stream, on the C++
side instead of the JS side.
This closes a hole where buffers that were temporarily created
in order to write strings with uncommon encodings (e.g. `hex`)
were passed to the native side without being set as `req.buffer`.
Fixes: https://github.com/nodejs/node/issues/8251
PR-URL: https://github.com/nodejs/node/pull/8252
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
