minor #28483 [HttpFoundation] don't override StreamedResponse::setNotModified() (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
----------

[HttpFoundation] don't override StreamedResponse::setNotModified()

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Alternative implementation to #27937, because `Response::setNotModified()` has been made final in 4.0 so we shouldn't override it.

Commits
-------

9ef7f7038d [HttpFoundation] don't override StreamedResponse::setNotModified()

src/Symfony/Component/HttpFoundation/StreamedResponse.php

@@ -123,6 +123,8 @@ class StreamedResponse extends Response
         if (null !== $content) {
             throw new \LogicException('The content cannot be set on a StreamedResponse instance.');
         }
+
+        $this->streamed = true;
     }
 
     /**
@@ -134,16 +136,4 @@ class StreamedResponse extends Response
     {
         return false;
     }
-
-    /**
-     * {@inheritdoc}
-     *
-     * @return $this
-     */
-    public function setNotModified()
-    {
-        $this->setCallback(function () {});
-
-        return parent::setNotModified();
-    }
 }