Update AuthenticatesUsers.php (#18746)

Additional checking for non string properties

Author: webmake

src/Illuminate/Foundation/Auth/AuthenticatesUsers.php

@@ -59,7 +59,8 @@ public function login(Request $request)
     protected function validateLogin(Request $request)
     {
         $this->validate($request, [
-            $this->username() => 'required', 'password' => 'required',
+            $this->username() => 'required|string',
+            'password' => 'required|string',
         ]);
     }