Handle floating point errors and trailing digits being removed by Pay U when checking returns

@edward @christianblais @j-mutter /cc @Shopify/payments 

`OffsitePayments::Return` also checks the amount that's coming back when it's confirming whether something is Ok or not - I missed updating this code last week to use the original gross amount that Pay U sends in.
So are saying we send them things like `2337.2960000000003`?