[Form] Added check for parent disabled status in Button form elements

The Button form element did not check for the parent disabled configuration status,
making them behave differently to all other form widgets.

src/Symfony/Component/Form/Button.php

@@ -321,7 +321,11 @@ class Button implements \IteratorAggregate, FormInterface
      */
     public function isDisabled()
     {
-        return $this->config->getDisabled();
+        if (null === $this->parent || !$this->parent->isDisabled()) {
+            return $this->config->getDisabled();
+        }
+
+        return true;
     }
 
     /**