Updates the Exception Handler in order to allow custom AuthenticationException message. (#21575)

Example use case: the user account gets disabled while the user is logged in the app and we want to logout the user and throw an exception with an appropriate message.
Before the PR, in the handler the custom message is overwritten before sending the response.

Author: aocneanu

src/Illuminate/Foundation/Exceptions/Handler.php

@@ -216,7 +216,7 @@ protected function prepareException(Exception $e)
     protected function unauthenticated($request, AuthenticationException $exception)
     {
         return $request->expectsJson()
-                    ? response()->json(['message' => 'Unauthenticated.'], 401)
+                    ? response()->json(['message' => $exception->getMessage()], 401)
                     : redirect()->guest(route('login'));
     }