Fixing rounding difference between main and confirm page

When doing a contribution, there is sometime a $0.01 difference between the amount as given in the main page (first step) and the confirmation page.

This is anoying but harmless in most cases but for Stripe, it refuses the transaction because the preauthorization happen in main page and the real payment is done after confirmation. If the amount are different, it will give an error.

Related to : dev/core#3714