scale_x_continuous: data is dropped even when na.value is set

[wch]

Examples:

dat <- data.frame(x=1:10)

ggplot(dat, aes(x=x, y=x, colour=x)) + geom_point()

# Shrink colour range: out-of-range points get na.value
ggplot(dat, aes(x=x, y=x, colour=x)) + geom_point() +
  scale_colour_continuous(limits=c(4,7), na.value="red")

# Shrink x range: out-of-range points are removed
# They should show up at x=5.5
ggplot(dat, aes(x=x, y=x, colour=x)) + geom_point() +
  scale_x_continuous(limits=c(4,7), na.value=5.5)
# Warning message:
# Removed 6 rows containing missing values (geom_point).