Store into cache using instance.id instead of requested id.

This helps ensure that the keys of the cache are the true and correct
data type and not just whatever the user provided. SQLite coerces
strings and ints.

voussoirkit/worms.py

@@ -455,7 +455,7 @@ class DatabaseWithCaching(Database, metaclass=abc.ABCMeta):
         instance = object_class(self, object_row)
 
         if object_cache is not None:
-            object_cache[object_id] = instance
+            object_cache[instance.id] = instance
 
         return instance