mailer: mailchimp bridge is throwing undefined index _id when setting message id in mandrill http transport

Author: monteiro

Mailchimp/Transport/MandrillHttpTransport.php

@@ -60,7 +60,7 @@ protected function doSendHttp(SentMessage $message): ResponseInterface
             throw new HttpTransportException(sprintf('Unable to send an email (code %s).', $result['code']), $response);
         }
 
-        $message->setMessageId($result['_id']);
+        $message->setMessageId($result[0]['_id']);
 
         return $response;
     }

MandrillHttpTransport.php

@@ -60,7 +60,7 @@ protected function doSendHttp(SentMessage $message): ResponseInterface
             throw new HttpTransportException(sprintf('Unable to send an email (code %s).', $result['code']), $response);
         }
 
-        $message->setMessageId($result['_id']);
+        $message->setMessageId($result[0]['_id']);
 
         return $response;
     }