Ticket #39066: 39066.2.diff

src/wp-includes/feed.php

@@ -702 +702 @@
         do_action_ref_array( 'wp_feed_options', array( &$feed, $url ) );
         $feed->init();
         $feed->set_output_encoding( get_option( 'blog_charset' ) );
-        $feed->handle_content_type();
 
+        // Don't change response headers if this is a REST, Ajax, or XMLRPC request.
+        if (
+                ( ! defined( 'REST_REQUEST' ) || ! REST_REQUEST )
+                && ( ! defined( 'XMLRPC_REQUEST' ) || ! XMLRPC_REQUEST )
+                && ! wp_doing_ajax()
+        ) {
+                $feed->handle_content_type();
+        }
+
         if ( $feed->error() )
                 return new WP_Error( 'simplepie-error', $feed->error() );