xep_0454: Ensure format_url returns a str

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

slixmpp/plugins/xep_0454/__init__.py

@@ -117,7 +117,7 @@ class XEP_0454(BasePlugin):
         """Helper to format a HTTPS URL to an AESGCM URI"""
         if not url.startswith('https://') or url.find('#') != -1:
             raise InvalidURL
-        url = 'aesgcm://' + url.removeprefix('https://') + '#' + fragment
+        return 'aesgcm://' + url.removeprefix('https://') + '#' + fragment
 
     async def upload_file(
         self,