Make Money::with less restritive to support interop with others implementations #113

 @Override
    public Money with(MonetaryOperator operator) { // <-- Can you return MonetaryAmount ?
        Objects.requireNonNull(operator);
        try {
            return Money.class.cast(operator.apply(this)); // <-- Can you relax this code ?
        } catch (MonetaryException e) {
            throw e;
        } catch (Exception e) {
            throw new MonetaryException("Operator failed: " + operator, e);
        }
    }