fix missing frontend js data

Hi!

There is a typo in the code for rex_browscap_get_frontend_data(). The php global is called $_COOKIES instead of $_COOKIE (http://php.net/manual/de/reserved.variables.cookies.php).

The commit fixes missing frontend data (http://www.redaxo.org/de/forum/addons-f30/rexbrowscap-addon-t14303-30.html#p104855)