Test currency string for validity

[jasonab]

Currently, when creating a CurrencyUnit from a String, the only way to know if that String is invalid is to catch an unchecked exception from the builder. It would be nice to have a boolean validator that would indicate whether or not a string is a valid currency code.

[jodastephen]

You can use this:

boolean valid = CurrencyUnit.registeredCurrencies().stream()
    .map(CurrencyUnit::getCode)
    .anyMatch(code -> code.equals("GBP"));

Adding methods for this would require potentially 6 new methods - one for each of method.