Make sure collections have trailing slashes (py-caldav is very pedantic about that)

crates/caldav/src/calendar_object/service.rs

@@ -50,6 +50,7 @@ impl<C: CalendarStore> ResourceService for CalendarObjectResourceService<C> {
     type PrincipalUri = CalDavPrincipalUri;
 
     const DAV_HEADER: &str = "1, 3, access-control, calendar-access";
+    const IS_COLLECTION: bool = false;
 
     async fn get_resource(
         &self,