allow negative items for REST API (thephpleague#231)

Author: SeoFood

src/Message/AbstractRestRequest.php

@@ -64,6 +64,11 @@ abstract class AbstractRestRequest extends \Omnipay\Common\Message\AbstractReque
 
     protected $referrerCode;
 
+    /**
+     * @var bool
+     */
+    protected $negativeAmountAllowed = true;
+    
     /**
      * @return string
      */