fix: use to_lowercase on /register/available username

src/client_server/account.rs

@@ -60,7 +60,7 @@ pub async fn get_register_available_route(
     body: Ruma<get_username_availability::Request<'_>>,
 ) -> ConduitResult<get_username_availability::Response> {
     // Validate user id
-    let user_id = UserId::parse_with_server_name(body.username.clone(), db.globals.server_name())
+    let user_id = UserId::parse_with_server_name(body.username.to_lowercase(), db.globals.server_name())
         .ok()
         .filter(|user_id| {
             !user_id.is_historical() && user_id.server_name() == db.globals.server_name()