Fixed code to pass unittests. Change script's is_valid method to correctly catch if it is checked on an object which is already deleted, as per #509.

src/server/models.py

@@ -48,6 +48,7 @@ class ServerConfig(WeakSharedMemoryModel):
     db_value = models.TextField(blank=True)
 
     objects = ServerConfigManager()
+    _is_deleted = False
 
     # Wrapper properties to easily set database fields. These are
     # @property decorators that allows to access these fields using