Feature/improve hmac validator (#260)

* Update util.py

* Update util.py

Author: antolo-arch

Adyen/util.py

@@ -37,6 +37,9 @@ def generate_notification_sig(dict_object, hmac_key):
 def is_valid_hmac_notification(dict_object, hmac_key):
     dict_object = dict_object.copy()
 
+    if 'notificationItems' in dict_object:
+        dict_object = dict_object['notificationItems'][0]['NotificationRequestItem']
+
     if 'additionalData' in dict_object:
         if dict_object['additionalData']['hmacSignature'] == "":
             raise ValueError("Must Provide hmacSignature in additionalData")