screencast: Make it possible to disable draw-cursor

drawing cursor is on by default, so the code:
  if (options['draw-cursor'])
    recorder.set_draw_cursor(options['draw-cursor']);
never lets you unset it.

Fix is to use 'draw-cursor' in options instead.

https://bugzilla.gnome.org/show_bug.cgi?id=744599

js/ui/screencast.js

@@ -99,7 +99,7 @@ const ScreencastService = new Lang.Class({
             recorder.set_pipeline(options['pipeline']);
         if (options['framerate'])
             recorder.set_framerate(options['framerate']);
-        if (options['draw-cursor'])
+        if ('draw-cursor' in options)
             recorder.set_draw_cursor(options['draw-cursor']);
     },