REST responses should have proper cache tags

I think this is critical, since changes to REST config will not appear in cached responses. Needs both the fix and tests

The simple fix from Wim & Berdir:

$response->headers->set('X-Drupal-Cache-Tags', implode(' ', $this->container->get('config.factory')->get('rest.settings')->getCacheTags()));

Let's see if this breaks anything. still needs tests.

Looks like this is blocking #2364011: [meta] External caches mix up response formats on URLs where content negotiation is in use so bumping status back up to critical. Agreed it probably wouldn't be critical in its own right though.

Looks ok, but the wording of this comment is a little unclear:

+    // Trigger a config save which should clear the page cache, so we should
+    // still get a cache miss for the same request.

So, the save doesn't clear the page cache per se as much as invalidating the cache tag and all caches that use it including the page cache.

Also we *now* not *still* get a cache miss? the last one was a hit.

Looks good. Entity cache tags is an important fix.

This issue addresses a critical bug and is allowed per https://www.drupal.org/core/beta-changes. Committed bf9a2f3 and pushed to 8.0.x. Thanks!

diff --git a/core/modules/rest/src/Tests/PageCacheTest.php b/core/modules/rest/src/Tests/PageCacheTest.php
index 70888a1..9cf2859 100644
--- a/core/modules/rest/src/Tests/PageCacheTest.php
+++ b/core/modules/rest/src/Tests/PageCacheTest.php
@@ -7,9 +7,6 @@
 
 namespace Drupal\rest\Tests;
 
-use Drupal\Component\Serialization\Json;
-use Drupal\rest\Tests\RESTTestBase;
-
 /**
  * Tests page caching for REST GET requests.
  *

Remove the unused use.

Awesome!