`execSync` not allow `encoding` to set explicitly to `buffer`

[zhanzhenzhen]

• Version: Tested on 5, 6
• Platform: Tested on OS X, Ubuntu
• Subsystem:

> child_process.execSync("ls",{encoding:"buffer"})
TypeError: Unknown encoding: buffer

[cjihrig]

Verified the issue.

One fix is cjihrig@ffb370f. Another fix would be supporting 'buffer' encoding in Buffer.prototype.toString().

EDIT: I'm not sure if the toString() approach makes sense though, as it would be giving back a Buffer, not a string.