Fixed #11200 -- Now use a `set` data structure for `GoogleMap` icons so that...

Fixed #11200 -- Now use a `set` data structure for `GoogleMap` icons so that they aren't repeated in rendered JavaScript.  Thanks to ludifan for ticket and initial patch.  


git-svn-id: http://code.djangoproject.com/svn/django/trunk@10865 bcc190cf-cafb-0310-a4f2-bffc1f526a37