Set challenge on UnauthorizedHttpException (dingo#1526)

the first parameter of Symfony\Component\HttpKernel\Exception\UnauthorizedHttpException can not be null in Symfony 4.

Author: MithrandirDK

src/Auth/Auth.php

@@ -110,7 +110,7 @@ protected function throwUnauthorizedException(array $exceptionStack)
         $exception = array_shift($exceptionStack);
 
         if ($exception === null) {
-            $exception = new UnauthorizedHttpException(null, 'Failed to authenticate because of bad credentials or an invalid authorization header.');
+            $exception = new UnauthorizedHttpException('dingo', 'Failed to authenticate because of bad credentials or an invalid authorization header.');
         }
 
         throw $exception;