PayPalModel Set Conditions updated

- passing null to setter would unset the value
- passing empty string would set the value as empty string
- passing 0 would set the value as 0
- Fixes #285

lib/PayPal/Common/PayPalModel.php

@@ -106,7 +106,7 @@ class PayPalModel
     public function __set($key, $value)
     {
         ModelAccessorValidator::validate($this, $this->convertToCamelCase($key));
-        if (!is_array($value) && $value == null) {
+        if (!is_array($value) && $value === null) {
             $this->__unset($key);
         } else {
             $this->_propMap[$key] = $value;