public function testToWebFormatWithMessageHeaders()
 {
     $email = new SendGrid\Email();
     $email->addMessageHeader('X-Sent-Using', 'SendGrid-API');
     $json = $email->toWebFormat();
     $headers = json_decode($json['headers'], TRUE);
     $this->assertEquals('SendGrid-API', $headers['X-Sent-Using']);
 }