drop_na() is removing attributes from columns

[xtimbeau]

drop_na() does not keep attributes of columns as shown below. I think it should.
thanks

ttbl <- tibble(col1 = c(1,2, NA, 2))
attr(ttbl$col1, "attr")<- "A value"
ttbl$col1
drop_na(ttbl)$col1

[hadley]

This looks like an easy fix — we just need to switch from [ to vec_slice()