Support 'tidyr::drop_na()'

[engie-paul-poncet]

Currently a tsibble is transformed into a tibble when applying tidyr::drop_na(), see for instance:

library(tidyr)
dt <- tsibble(x = c("A", "A", "B"), y = c(1, 2, 1), key = "x", index = "y")
class(drop_na(dt))

Could a tsibble be returned instead?