[HttpKernel] fixed exception constructor signature

src/Symfony/Component/HttpKernel/Exception/ForbiddenHttpException.php

@@ -18,7 +18,7 @@ namespace Symfony\Component\HttpKernel\Exception;
  */
 class ForbiddenHttpException extends HttpException
 {
-    public function __construct($message = '', $code = 0, \Exception $previous)
+    public function __construct($message = '', $code = 0, \Exception $previous = null)
     {
         if (!$message) {
             $message = 'Forbidden';

src/Symfony/Component/HttpKernel/Exception/NotFoundHttpException.php

@@ -18,7 +18,7 @@ namespace Symfony\Component\HttpKernel\Exception;
  */
 class NotFoundHttpException extends HttpException
 {
-    public function __construct($message = '', $code = 0, \Exception $previous)
+    public function __construct($message = '', $code = 0, \Exception $previous = null)
     {
         if (!$message) {
             $message = 'Not Found';

src/Symfony/Component/HttpKernel/Exception/UnauthorizedHttpException.php

@@ -18,7 +18,7 @@ namespace Symfony\Component\HttpKernel\Exception;
  */
 class UnauthorizedHttpException extends HttpException
 {
-    public function __construct($message = '', $code = 0, \Exception $previous)
+    public function __construct($message = '', $code = 0, \Exception $previous = null)
     {
         if (!$message) {
             $message = 'Unauthorized';