Creating attribute based discounts in Zen Cart – bugfix

An issue was discovered in Zen Cart Big Chooser which affects people trying to create discounts based on products with specfic attribute settings.  As an example,

 $this->add_condition("Buy 22 boxed edition, get download edition free", true);
     $this->set_constraint(PROD, 22, 1);
     $this->set_constraint_attrfilter(5, 13);
     $this->set_discount(PROD, 22, 1, "%", 100);
     $this->set_discount_attrfilter(5, 10);

This discount allows to to offer product 22 (with attribute 5 set to value 10) free when product 22 (with attribute 5 set to 13) is purchased.  This style of discounting works correctly in Big Chooser 1.2.15.