paybox_hidden_fields helper trouble

As I wrote here : https://github.com/slainer68/paybox_system/issues/2, the rails helper method escape strings values. If these values contains special chars, the HMAC returned by hash_form_fields_from is not calculated on same values. 
We could prefer to change hash_form_fields_from and make an escape on each value, but I am afraid i could change the 'PBX_RETOUR' and other fields needed :-(

Nicolas