optimize Buffer#toString()
optimize Buffer#toString()

From
https://github.com/nodejs/io.js/commit/8350f3a3a291fcd19df3261c3db3fe07c
7590bb7

feross/buffer,nolanlawson/buffer,LinusU/buffer,LiuDeng/buffer,SAGV/buffer,LinusU/buffer,kawanet/buffer,feross/buffer,nolanlawson/buffer,loon3/buffer,beni55/buffer,kawanet/buffer,btd/buffer,feross/buffer,LiuDeng/buffer,loon3/buffer,beni55/buffer,btd/buffer,SAGV/buffer