code: func (b *ByteBuf) WriteByteBuf(from *ByteBuf) error {
  	size := from.Readable()
  	b.Expansion(size)
  	copy(b.buf[b.writerIndex:b.writerIndex+size], from.buf[from.readerIndex:from.writerIndex])
  	b.writerIndex += size
  	from.readerIndex = from.writerIndex
  	return nil
  }
docs: // WriteByteBuf write all readable data to this buf