group_by %.% mutate does not preserve tzone for POSIXct

[ateucher]

This was fixed for summarise (#252), but still occurs with mutate (only on a grouped df). This is with the current development version of dplyr:

require(lubridate)
require(dplyr)

test.df <- structure(
  list(ID = c(rep(1,20),rep(2,20)),
       Date = structure(c(-247320000, -246196800, -245073600, -243864000,
                            -242654400, -241444800, -126273600, -123595200,
                            -121176000, -118497600, 1359385200, 1359388800,
                            1359392400, 1359396000, 1359399600, 1359403200,
                            1359406800, 1359410400, 1359414000, 1359417600,
                            55598400, 56116800, 58881600, 62078400, 64756800,
                            67348800, 69854400, 72964800, 76161600, 79012800,
                            1358589600, 1358676000, 1358762400, 1358848800,
                            1358935200, 1359021600, 1359108000, 1359194400,
                            1359280800, 1359367200), tzone = "GMT",
                          class = c("POSIXct", "POSIXt")),
       Val=rnorm(40)),
  .Names = c("ID", "Date", "Val"), row.names = c(NA, 40L),
  class = "data.frame")

out.df <- group_by(test.df, ID, fl_date=floor_date(Date, unit="month")) %.%
  mutate(length_x=n(), summary_date = 
           if (n() < 5) round_date(Date, "month") else floor_date(Date, "day"))

attr(out.df$summary_date, "tzone") # This is null

This works:

out2.df <-test.df %.% mutate(summary_date = round_date(Date, "month"))
attr(out2.df$summary_date, "tzone") # "GMT"

I left this as a comment in #252 after it was closed, so I thought it may have been lost - my apologies if it's already been captured.