Handle PayU notifications missing trailing 0 on two digit amounts

@aprofeit @j-mutter /cc @Shopify/payments 

The fix for PayU's floating point problems caused problems because it also changed the format of `63.0` in notifications to be `63.00` - which made checksums break. This change goes back to using their original value for notification checksums, but leaves `gross` as the reformatted value.
:trollface: