buffer: allow .write() offset to be at buffer end

Do not throw if the offset passed to `buf.write()` points
to the end of the buffer.

Fixes: https://github.com/nodejs/node/issues/8127
PR-URL: https://github.com/nodejs/node/pull/8154


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>