fix the disorder not ignored error

[wuYin]

Issue

func main() {
	c, err := beanstalk.Dial("tcp", "127.0.0.1:11300")
	if err != nil {
		log.Fatal(err)
	}

	c.TubeSet = *beanstalk.NewTubeSet(c) // ignore "default" tube
	if _, _, err = c.Reserve(1 * time.Second); err != nil {
		fmt.Println(err) // reserve-with-timeout: not ignored // ok
	}

	if _, err = c.Tube.Stats(); err != nil {
		fmt.Println(err) // stats-tube: timeout // unexpected disordered "timeout" 
	}
}

Cause

In current implementation, there is no function ignore for tubeSet or watch for tube, only adjust watching tube list before sending reserve cmd, so we sent ignore but never read NOT_IGNORED

Modification

• Simulated return not ignored error if user ignore all tubes.
• Discard NOT_IGNORED error text line when reading response.