clicking back button on order review page results in 0.00 in donation checkout pane

Issue Summary

When using uc_donate module, you can add an additional donation to the order on the checkout page. If the user then clicks forward to the order review page and subsequently hits the 'back' button, or if credit card validation fails (uc_credit module), then the user is returned to the checkout page with all fields filled in except the donation amount which is reset to 0.00. The attached patch fixes this.