title: Redirection and form's processing totally ignored by Symfony
body: <p>Symfony doesn't redirect even when I put the redirection right after the start of the it's processing <em>(Minimalist example)</em> : </p>
  
  <pre><code>if($formPhoto-&gt;isSubmitted() &amp;&amp; $formPhoto-&gt;isValid()){
     $this-&gt;redirectToRoute('app_acc');
  }
  </code></pre>
  
  <p>It seems like Symfony never enter in the processing. However, the data are send properly to the database, and when I put a <code>dump('Hello');</code> instead of the <code>$this-&gt;redirectToRoute('app_acc');</code> I see it on the refreshed page, so Symfony enter in it.</p>
  
  <p>In my routes.yaml I have this : <em>(I know it works well because used elsewhere)</em> </p>
  
  <pre><code>app_acc:
  path: /{_locale}
  controller: App\Controller\AccueilController::index
  defaults:
      _locale: fr
  requirements:
      _locale: en|fr
  </code></pre>
  
  <p>I don't understand why does Symfony ignore my redirections &amp; some of my processings ?
  Does someone has an idea ? I'm starting to go crazy. 
  Thank you </p>