Issue #2537602 by hchonov, dawehner: JQuery ajax GET requests result in "406 Not Acceptable"

core/lib/Drupal/Core/ContentNegotiation.php

@@ -37,10 +37,6 @@ class ContentNegotiation {
       return $request->query->get('_format');
     }
 
-    if ($request->isXmlHttpRequest()) {
-      return 'ajax';
-    }
-
     // Do HTML last so that it always wins.
     return 'html';
   }

core/modules/simpletest/src/WebTestBase.php

@@ -1555,7 +1555,15 @@ abstract class WebTestBase extends TestBase {
     if (!isset($options['query'][MainContentViewSubscriber::WRAPPER_FORMAT])) {
       $options['query'][MainContentViewSubscriber::WRAPPER_FORMAT] = 'drupal_ajax';
     }
-    return Json::decode($this->drupalGet($path, $options, $headers));
+    return Json::decode($this->drupalGetXHR($path, $options, $headers));
+  }
+
+  /**
+   * Requests a Drupal path as if it is a XMLHttpRequest.
+   */
+  protected function drupalGetXHR($path, array $options = array(), array $headers = array()) {
+    $headers[] = 'X-Requested-With: XMLHttpRequest';
+    return $this->drupalGet($path, $options, $headers);
   }
 
   /**

core/modules/system/src/Tests/Ajax/DialogTest.php

@@ -94,6 +94,10 @@ class DialogTest extends AjaxTestBase {
     $this->drupalGet('ajax-test/dialog-contents');
     $this->assertRaw($dialog_contents, 'Non-JS modal dialog page present.');
 
+    // Check that requesting a modal dialog with XMLHttpRequest goes to a page.
+    $this->drupalGetXHR('ajax-test/dialog-contents');
+    $this->assertRaw($dialog_contents, 'Modal dialog page on XMLHttpRequest present.');
+
     // Emulate going to the JS version of the page and check the JSON response.
     $ajax_result = $this->drupalGetAjax('ajax-test/dialog-contents', array('query' => array(MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_modal')));
     $this->assertEqual($modal_expected_response, $ajax_result[3], 'Modal dialog JSON response matches.');

core/tests/Drupal/Tests/Core/ContentNegotiationTest.php

@@ -73,7 +73,7 @@ class ContentNegotiationTest extends UnitTestCase {
     $request = new Request();
     $request->headers->set('X-Requested-With', 'XMLHttpRequest');
 
-    $this->assertSame('ajax', $this->contentNegotiation->getContentType($request));
+    $this->assertSame('html', $this->contentNegotiation->getContentType($request));
   }
 
 }