fixed a bug in getResponse.lm

There's a little bug in `getResponse.lm()`.

```s
library(fpp)
creditlog <- data.frame(score=credit$score,
	               log.savings=log(credit$savings+1),
		       log.income=log(credit$income+1),
		       log.address=log(credit$time.address+1),
		       log.employed=log(credit$time.employed+1),
		       fte=credit$fte, single=credit$single)
fit <- lm(log(score) ~ log.savings + log.income +
		       log.address + log.employed, data=creditlog[1:480, ])
fcasts <- forecast(fit, newdata = creditlog[481:500, ], lambda = 0)  
# Error due to getResponse
```