core (persistent dict): added setdefault method

src/memory/persistent.py

@@ -118,6 +118,13 @@ class PersistentDict(object):
     def get(self, key, default=None):
         return self._cache.get(key, default)
 
+    def setdefault(self, key, default):
+        try:
+            return self._cache[key]
+        except:
+            self.__setitem__(key, default)
+            return default
+
     def force(self, name):
         """Force saving of an attribute to storage
         @return: deferred fired when data is actually saved"""