message: Validator._guardExtended: deleted restriction for getMessage attribute.
  This caused severe problems when using multilanguage as it overrided the translation for the current locale.
  Now this implementation is compliant with the documentation wich says "can contain a getMessage method", but it's not required.
diff: @@ -572,12 +572,6 @@ export default class Validator {
  `Extension Error: The validator '${name}' must be a function or have a 'validate' method.`
  );
  }
  -
  - if (!isCallable(validator.getMessage) && typeof validator.getMessage !== 'string') {
  - throw createError(
  - `Extension Error: The validator '${name}' object must have a 'getMessage' method or string.`
  - );
  - }
  }
  /**
  