Fix len method of AbstractStorage

Author: angelakis

src/oidcmsg/storage/__init__.py

@@ -51,7 +51,7 @@ def __repr__(self):
         return self.__str__()
 
     def __len__(self):
-        return len(self.storage())
+        return len(self.storage)
 
     def __contains__(self, k):
         return self.storage.__contains__(k)