Reduced CookieStorage.max_cookie_size to 75% of 4K to be nice to Internet Explorer

Internet Explorer, up to at least version 7, allows only 4K cookie data *per
domain*, rather than per cookie, so we ensure that stored messages leave
some room for other cookies.




git-svn-id: http://code.djangoproject.com/svn/django/trunk@12287 bcc190cf-cafb-0310-a4f2-bffc1f526a37