fix #495

[shrektan]

styleInterval() and styleEqual() now generates the correct callback for Date and Datetime values.

Closes #495

Example

library(DT)
dates <- as.Date(c("2018-01-01","2019-01-01"))
times <- as.POSIXct(c("2018-01-01 15:00", "2018-01-01 20:00"), tz = "UTC")
tbl <- 
  data.frame(
    Date = dates,
    Date2 = dates,
    Time = times,
    Time2 = times,
    Time3 = times,
    Time4 = times
  )
tbl %>%
  datatable() %>%
  formatStyle(columns = c("Date", "Date2"),
              backgroundColor = styleInterval(as.Date("2018-02-02"), c("#FB717E", "#89EC6A"))) %>%
  formatStyle(columns = c("Time", "Time2"),
              backgroundColor = styleInterval(as.POSIXct(c("2018-01-01 18:00"), tz = "UTC"), c("#FB717E", "#89EC6A"))) %>%
  formatStyle(columns = c("Time3"),
              backgroundColor = styleInterval(as.POSIXct(c("2018-01-02 02:00"), tz = "PRC"), c("#FB717E", "#89EC6A"))) %>%
  formatStyle(columns = c("Time4"),
              backgroundColor = styleEqual(times, c("#FB717E", "#89EC6A"))) %>%
  formatDate(columns = c("Date2", "Time2"), c("toDateString", "toTimeString"))

[yihui]

Excellent. Thanks!