maintain tbl_df input

[mdsumner]

small change to ensure tibble-in, tibble-out

  f <- system.file("gpkg/nc.gpkg", package = "sf", mustWork = TRUE)

x <- sf::read_sf(f)
o <- rmapshaper::ms_simplify(x)
class(x)
#> [1] "sf"         "tbl_df"     "tbl"        "data.frame"
class(o)
#> [1] "sf"         "tbl_df"     "tbl"        "data.frame"

^{Created on 2019-11-06 by the reprex package (v0.3.0)}

I'm motivated by this because I rely on the behaviour of tibble in several places, I don't think it can cause problems - the test for it's a tibble is very specific (could be made more general in future if needed).

The only difference is the class attribute, as in if you compare sf::st_as_sf(tibble::as_tibble(rmapshapher::ms_simplify(x))) with current behaviour, you get an identical object with this PR.

[ateucher]

Thanks @mdsumner, and sorry for the long wait!