`first` and `last` fail for factors

[tmalsburg]

> first((1:3)[integer(0)])
[1] NA
> first(factor(1:3)[integer(0)])
Error: Don't know how to generate default for object of class factor

I would expect to get an NA for factors, too. Same problem for last.

[krlmlr]

Thanks, confirmed. Internally, we check is.vector(factor(letters)[0]) which is FALSE but should not be.

Would you like to contribute a testthat test?

[hadley]

Similar problem for POSIXct and Date. Fix should be to make default_missing a generic. I'll do that tomorrow.