Make a value reactive if it is not already reactive.

This is used to allow parameters to Shiny functions to be optionally reactive.

ensure_reactive(item, name = NULL, env = NULL, default = function()
  stop("Missing ", name))

Arguments

item	Thing to ensure reactive, either an already reactive value, or a plain value, or NULL.
name	If item is NULL, env[[name]]() will be used.
env	If item is NULL, env[[name]]() will be used.
default	If item is NULL, and name is not in env, this reactive value will be used.

A Shiny reactive value.