Follow up to #3996: fix front-end checkbox label updating logic

[cpsievert]

#3996 changed updateCheckboxInput(label = ) to send a object instead of a string to the front-end; however, the front-end receiveMessage() wasn't updated to handle this new data type.

For a minimal example, this now updates the label correctly (before this PR, it would render as [object Object]

library(shiny)

ui <- fluidPage(
  checkboxInput("foo", "label")
)

server <- function(input, output, session) {
  observe({
    updateCheckboxInput(session, "foo", label = HTML("<i>new label</i>"))
  })
}

shinyApp(ui = ui, server = server)