public function test500or503()
 {
     try {
         $client = new Client($this->configParams);
         $url = 'https://www.justcharge.me/OffAmazonPayments_Sandbox/2013-01-01';
         $client->setMwsServiceUrl($url);
         $this->callPrivateMethod($client, 'invokePost', null);
     } catch (\Exception $expected) {
         $this->assertRegExp('/Maximum number of retry attempts./i', strval($expected));
     }
 }