Remove Buffer._charsWritten (parity with 0.11)
Remove Buffer._charsWritten (parity with 0.11)

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