child_process: allow buffer encoding in spawnSync

When the 'buffer' encoding is passed to spawnSync(), an exception
is thrown in Buffer's toString() method because 'buffer' is not
a valid encoding there. This commit special cases the 'buffer'
encoding.

Fixes: https://github.com/nodejs/node/issues/6930
PR-URL: https://github.com/nodejs/node/pull/6939


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>