code: public BigMoney plusRetainScale(BigDecimal amountToAdd, RoundingMode roundingMode) {
          MoneyUtils.checkNotNull(amountToAdd, "Amount must not be null");
          if (amountToAdd.compareTo(BigDecimal.ZERO) == 0) {
              return this;
          }
          BigDecimal newAmount = amount.add(amountToAdd);
          newAmount = newAmount.setScale(getScale(), roundingMode);
          return BigMoney.of(currency, newAmount);
      }
docs: Returns a copy of this monetary value with the amount added retaining
  the scale by rounding the result.
  <p>
  The scale of the result will be the same as the scale of this instance.
  For example,'USD 25.95' plus '3.021' gives 'USD 28.97' with most rounding modes.
  <p>
  This instance is immutable and unaffected by this method.
  
  @param amountToAdd  the monetary value to add, not null
  @param roundingMode  the rounding mode to use to adjust the scale, not null
  @return the new instance with the input amount added, never null