fix: output is not ordered by pubDate (#28)

related issues: #26

Author: LeslieLeung

src/main/java/run/halo/feed/FeedServiceImpl.java

@@ -151,7 +151,7 @@ private Mono<ServerResponse> postListResultToXmlServerResponse(
         FeedContext feedContext, RSS2 rss2) {
         return postListResult
             .flatMapIterable(ListResult::getItems)
-            .flatMap(post -> {
+            .concatMap(post -> {
                 // Create item
                 var permalink = post.getStatusOrDefault().getPermalink();
                 if (permalink != null) {