Use a little more robust method of checking instances

Author: kriszyp

lib/validate.js

@@ -208,7 +208,7 @@ var validate = exports._validate = function(/*Any*/instance,/*Object*/schema,/*O
 
 			for(var i in objTypeDef){ 
 				if(objTypeDef.hasOwnProperty(i) && i != '__proto__' && i != 'constructor'){
-					var value = instance[i];
+					var value = instance.hasOwnProperty(i) ? instance[i] : undefined;
 					// skip _not_ specified properties
 					if (value === undefined && options.existingOnly) continue;
 					var propDef = objTypeDef[i];