Made tslm() have identical object structure as lm()

Removed fit$x and fit$data, resolves dwtest() issue #329.

```R
library(fpp)
fit1 <- tslm(sales ~ advert, data=advert)
fit2 <- lm(sales ~ advert, data=advert)

dwtest(fit1)
dwtest(fit2)

fc1 <- forecast(fit1, newdata=data.frame(advert=1:7))
fc2 <- forecast(fit2, newdata=data.frame(advert=1:7))

plot(fc1)
autoplot(fc1)

plot(fc2)
autoplot(fc2)
```