Preserve NA when converting to strings

[jeroen]

I may be overlooking something, but why are NAs not preserved?

[krlmlr]

Good point. We need an NA string for format(), but maybe this still works with your patch? Existing tests seem to pass, would you like to add an explicit test for as.character() and maybe format()?

[jimhester]

FWIW the current behavior was the ultimate cause of a reported readr bug tidyverse/readr#715, which should be fixed once this is merged.

[krlmlr]

Thanks!