minor #5972 Add isSubmitted call (DanielSiepmann)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #5972).

Discussion
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Add isSubmitted call

Streamline with http://symfony.com/doc/current/book/forms.html#handling-form-submissions
Following best practice: http://symfony.com/doc/current/best_practices/forms.html

Commits
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d8c93e2 Add isSubmitted call

Author: wouterj

book/forms.rst

@@ -231,7 +231,7 @@ controller::
 
         $form->handleRequest($request);
 
-        if ($form->isValid()) {
+        if ($form->isSubmitted() && $form->isValid()) {
             // ... perform some action, such as saving the task to the database
 
             return $this->redirect($this->generateUrl('task_success'));