doing ReadCodeLine in Close() @ ReadCloser

If I get a ReadCloser From Retr and immediately close it but not read anything, the code line will be messed up.

The code below can reappear this problem.
```{go}
	r, err := c.Retr("/123.txt")
	if err != nil {
		log.Fatalln(err)
	}
	io.Copy(ioutil.Discard, r)
	r.Close()

	r, err = c.Retr("/123.txt")
	if err != nil {
		log.Fatalln(err) //ok
	}
	r.Close()  //close it but not read anything

	r, err = c.Retr("/123.txt")
	if err != nil {
		log.Fatalln(err) // messed up
	}
	r.Close()
```