Round down on original payment and credit reimbursement types. 

* Add a bit of leniency in the edge-of-edge cases on multiple reimbursement types and rounding issues. Context in the comment

This was also especially hard to write a test, since spree only has one fully-functioning reimbursement type (Credits are only half-there due to no implementation of store credit in spree), and the fact that this super-edge case only happens with multiple reimbursement types in conjunction with very specific price conditions. Perhaps we could write a specific test in spree-backend?