fix: cache string

music_kraken/connection/cache.py

@@ -71,6 +71,7 @@ class Cache:
         for c in self.cached_attributes:
             d = c.__dict__
             for key in self._time_fields:
+                if not isinstance(d[key], str):
                     d[key] = d[key].isoformat()
 
             _json.append(d)