Cast value as string to pass new Validator.js string requirement

lib/instance-validator.js

@@ -254,12 +254,14 @@ InstanceValidator.prototype._invokeCustomValidator = Promise.method(function(val
  */
 InstanceValidator.prototype._invokeBuiltinValidator = Promise.method(function(value, test, validatorType, field) {
   var self = this;
+  // Cast value as string to pass new Validator.js string requirement
+  var valueString = String(value);
   // check if Validator knows that kind of validation test
   if (typeof validator[validatorType] !== 'function') {
     throw new Error('Invalid validator function: ' + validatorType);
   }
   var validatorArgs = self._extractValidatorArgs(test, validatorType, field);
-  if (!validator[validatorType].apply(validator, [value].concat(validatorArgs))) {
+  if (!validator[validatorType].apply(validator, [valueString].concat(validatorArgs))) {
   // extract the error msg
     throw new Error(test.msg || 'Validation ' + validatorType + ' failed');
   }