merged branch jfsimon/issue-3896 (PR #4029)

Commits
-------

686653a [HttpKernel] Fixed wache vary write (fixes #3896).

Discussion
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[HttpKernel] Fixed cache vary write (fixes #3896).

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: #3896

Same as https://github.com/symfony/symfony/pull/4001, but for write method.

src/Symfony/Component/HttpKernel/HttpCache/Store.php

@@ -167,7 +167,7 @@ class Store implements StoreInterface
         $entries = array();
         $vary = $response->headers->get('vary');
         foreach ($this->getMetadata($key) as $entry) {
-            if (!isset($entry[1]['vary'])) {
+            if (!isset($entry[1]['vary'][0])) {
                 $entry[1]['vary'] = array('');
             }