do() loses ordering of ordered factors

[ateucher]

When executing an arbitrary function on a grouped data frame using do(), ordered factors lose their order:

library(dplyr)
l <- c("a", "b", "c", "d")
id <- factor(c("a", "c", "d"), levels = l, ordered = TRUE)
df <- data.frame(id = rep(id, 2), val = rnorm(6))

levels(df$id)
## [1] "a" "b" "c" "d"

is.ordered(df$id)
## [1] TRUE

out <- group_by(df, id) %>% 
  do(na.omit(.))

levels(out$id) # Levels are retained
## [1] "a" "b" "c" "d"

is.ordered(out$id) # Loses ordering
## [1] FALSE

[hadley]

This is probably a bind_rows() problem