set_conditional_cache_control!

# File actionpack/lib/action_dispatch/http/cache.rb, line 115
        def set_conditional_cache_control!
          return if self["Cache-Control"].present?

          control = @cache_control

          if control.empty?
            headers["Cache-Control"] = DEFAULT_CACHE_CONTROL
          elsif @cache_control[:no_cache]
            headers["Cache-Control"] = "no-cache"
          else
            extras  = control[:extras]
            max_age = control[:max_age]

            options = []
            options << "max-age=#{max_age.to_i}" if max_age
            options << (control[:public] ? "public" : "private")
            options << "must-revalidate" if control[:must_revalidate]
            options.concat(extras) if extras

            headers["Cache-Control"] = options.join(", ")
          end
        end