child_process: preserve argument type

A previous fix for a `maxBuffer` bug resulted in a change to the
argument type for the `data` event on `child.stdin` and `child.stdout`
when using `child_process.exec()`.

This fixes the `maxBuffer` bug in a way that does not have that side
effect.

PR-URL: https://github.com/nodejs/node/pull/7391


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>
Fixes: https://github.com/nodejs/node/issues/7342
Refs: https://github.com/nodejs/node/issues/1901