protected function prepareSubscriber(Subscriber $subscriber)
 {
     $subscriber->getEmail()->willReturn('*****@*****.**');
     $subscriber->getMergeTags()->willReturn(['FIRSTNAME' => 'Charles']);
 }
 public function subscribe($listId, Subscriber $subscriber)
 {
     $this->mailchimp->lists->subscribe($listId, ['email' => $subscriber->getEmail()], $subscriber->getMergeTags(), 'html', false, true);
 }