public function signRequest(RequestInterface $request, CredentialsInterface $credentials)
 {
     if ($request instanceof EntityEnclosingRequestInterface && $request->getBody()) {
         $request->setHeader('X-Amz-Content-Sha256', EntityBody::getHash($request->getBody(), 'sha256'));
     } else {
         $request->setHeader('X-Amz-Content-Sha256', hash('sha256', ''));
     }
     parent::signRequest($request, $credentials);
 }