Returns False on Validation for Access Modifier on generic PPModel object

- Fixes #171 Issue

lib/PayPal/Validation/ModelAccessorValidator.php

@@ -26,8 +26,12 @@ class ModelAccessorValidator
         if ($mode != 'disabled') {
             //If the mode is disabled, bypass the validation
             foreach (array('set' . $attributeName, 'get' . $attributeName) as $methodName) {
+                if (get_class($class) == "PayPal\\Common\\PPModel") {
+                    // Silently return false on cases where you are using PPModel instance directly
+                    return false;
+                }
                 //Check if both getter and setter exists for given attribute
-                if (!method_exists($class, $methodName)) {
+                elseif (!method_exists($class, $methodName)) {
                     //Delegate the error based on the choice
                     $className = is_object($class) ? get_class($class) : (string)$class;
                     $errorMessage = "Missing Accessor: $className:$methodName. Please let us know by creating an issue at https://github.com/paypal/PayPal-PHP-SDK/issues";