[WC 2.3] Prevents errors on mini cart when debug mode is activated

This message will broken the mini cart ajax response:

```
[12-Dec-2014 12:11:13 UTC] PHP Notice:  The add_to_cart_fragments filter is <strong>deprecated</strong> since version ! Use woocommerce_add_to_cart_fragments instead. in /var/www/woocommerce-dev/wp-includes/functions.php on line 3378
```