Fix URL-encoded bookmarking with date/date-time sliders

[wch]

This fixes #1600.

Test app:

library(shiny)

now <- Sys.time()
today <- Sys.Date()

ui <- function(request) {fluidPage(
  sliderInput("datetime", "Date-time",
    min = now, max = now + 10*3600, value = now + c(3, 4)*3600
  ),
  sliderInput("date", "Date",
    min = today, max = today + 10, value = today + 5
  ),
  verbatimTextOutput("values"),
  bookmarkButton()
)}

server <- function(input, output, server) {
  output$values <- renderText({
    paste(
      paste(input$datetime, collapse = ", "),
      input$date,
      sep = "\n"
    )
  })
}

shinyApp(ui, server, enableBookmarking = "url")