only in patch2:
unchanged:
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -1189,7 +1189,10 @@ protected function drupalGet($path, array $options = array(), array $headers = a
    * Retrieve a Drupal path or an absolute path and JSON decode the result.
    */
   protected function drupalGetAJAX($path, array $options = array(), array $headers = array()) {
+    // Emulate the headers that jQuery sends.
     $headers[] = 'X-Requested-With: XMLHttpRequest';
+    $headers[] = 'Accept: application/vnd.drupal-ajax, */*; q=0.01';
+
     return drupal_json_decode($this->drupalGet($path, $options, $headers));
   }
 
@@ -1410,7 +1413,10 @@ protected function drupalPostAJAX($path, $edit, $triggering_element, $ajax_path
     }
     $content = $this->content;
     $drupal_settings = $this->drupalSettings;
+
+    // Emulate the headers that jQuery sends.
     $headers[] = 'X-Requested-With: XMLHttpRequest';
+    $headers[] = 'Accept: application/vnd.drupal-ajax, */*; q=0.01';
 
     // Get the Ajax settings bound to the triggering element.
     if (!isset($ajax_settings)) {