Support CVV with leading zeroes

Support CVV with leading zeroes
Trim whitespace from string values
> All objects have toString() so the use-case doesn’t apply to StringOrNull - every Object can be rendered as a String.

`StringOrNull` is a bit smarter than simply calling `toString` - it understands if the returned XML payload represents a `null` object in the Recurly format.

Note that these methods are not supposed to do any validation, they are simply transforming the XML output sent from Recurly to the Java type expected by the model (the setters are called during deserialization).

If you would like to add client-side validation, which could be a good thing, we should probably do it somewhere else. Maybe `RecurlyObject` could have a `validate` method that individual models could override (and we would do the CVV check there). `validate` would then be called before serialization.