bug #34385 Avoid empty "If-Modified-Since" header in validation reque…

…st (mpdude)

This PR was squashed before being merged into the 3.4 branch (closes #34385).

Discussion
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Avoid empty "If-Modified-Since" header in validation request

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

Just noticed that when a response has been cached that is `public` and has an `maxAge` but does _not_ provide `Last-Modified`, the validation subrequest will have an empty `If-Modified-Since` header value.

Commits
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960faef Avoid empty \"If-Modified-Since\" header in validation request

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

@@ -352,7 +352,9 @@ protected function validate(Request $request, Response $entry, $catch = false)
         }
 
         // add our cached last-modified validator
-        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified'));
+        if ($entry->headers->has('Last-Modified')) {
+            $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified'));
+        }
 
         // Add our cached etag validator to the environment.
         // We keep the etags from the client to handle the case when the client

src/Symfony/Component/HttpKernel/Tests/HttpCache/HttpCacheTest.php

@@ -859,6 +859,7 @@ public function testValidatesCachedResponsesUseSameHttpMethod()
     public function testValidatesCachedResponsesWithETagAndNoFreshnessInformation()
     {
         $this->setNextResponse(200, [], 'Hello World', function ($request, $response) {
+            $this->assertFalse($request->headers->has('If-Modified-Since'));
             $response->headers->set('Cache-Control', 'public');
             $response->headers->set('ETag', '"12345"');
             if ($response->getETag() == $request->headers->get('IF_NONE_MATCH')) {