[-] Fix the PS_ALLOW_ACCENTED_CHARS_URL bug

In Prestashop < 1.5.3 utf8 chars was allowed in URL (it was a good thing btw)
In Prestashop >= 1.5.3 no more but rewrited URL are not modified 
so if you simply do
```PHP
$p = new Product((int) $id_product, (int) $id_lang);
$p->save();
```
you can have an exception : "Property Product->link_rewrite is not valid"