Fix arg signature

src/Composer/Util/RemoteFilesystem.php

@@ -308,7 +308,7 @@ class RemoteFilesystem
         }
     }
 
-    protected function promptAuthAndRetry($httpStatus, $reason)
+    protected function promptAuthAndRetry($httpStatus, $reason = null)
     {
         if ($this->config && in_array($this->originUrl, $this->config->get('github-domains'), true)) {
             $message = "\n".'Could not fetch '.$this->fileUrl.', enter your GitHub credentials '.($httpStatus === 404 ? 'to access private repos' : 'to go over the API rate limit');