Cacheable HTML and JSON responses handled differently when HTTP 4xx occurs

Closes #3451483

core/modules/dynamic_page_cache/tests/dynamic_page_cache_test/src/DynamicPageCacheTestController.php

@@ -2,6 +2,7 @@
 
 namespace Drupal\dynamic_page_cache_test;
 
+use Drupal\Core\Cache\CacheableJsonResponse;
 use Drupal\Core\Cache\CacheableResponse;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\user\Entity\User;
@@ -25,6 +26,16 @@ public function response() {
     return new Response('foobar');
   }
 
+  /**
+   * A route returning a cacheable JSON response.
+   *
+   * @return \Drupal\Core\Cache\CacheableJsonResponse
+   *   The response object.
+   */
+  public function cacheableJsonResponse(): CacheableJsonResponse {
+    return new CacheableJsonResponse(['OK']);
+  }
+
   /**
    * A route returning a CacheableResponse object.
    *