…en setting message id (monteiro)This PR was merged into the 4.4 branch.Discussion----------[Mailer][Mailchimp Bridge] Throwing undefined index _id when setting message id| Q             | A| ------------- | ---| Branch?       | master for features / 4.4 or 5.0 for bug fixes <!-- see below -->| Bug fix?      | yes| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->| Tickets       | (no ticket created)| License       | MIT<!--Replace this notice by a short README for your feature/bugfix. This will help peopleunderstand your PR and can be used as a start for the documentation.Additionally (seehttps://symfony.com/roadmap): - Always add tests and ensure they pass. - Never break backward compatibility (seehttps://symfony.com/bc). - Bug fixes must be submitted against the lowest maintained branch where they apply   (lowest branches are regularly merged to upper ones so they get the fixes too.) - Features and deprecations must be submitted against branch master.-->When I set the DSN:`MAILER_DSN=mandrill://KEY@default`and try to send an email:```php$reportEmail = (new Email())            ->subject('this is a test')            ->addFrom('test@test.com')            ->addTo('test-2@test.com')            ->text('this is just a test.');$mailer->send($reportEmail);```it returns an exception:<img width="1059" alt="Captura de ecrã 2019-11-25, às 14 31 59" src="https://user-images.githubusercontent.com/74459/69549352-0a8e6480-0f91-11ea-8d6e-ecb1ecb8caa5.png">Stacktrace:```ErrorException:Notice: Undefined index: _id  at /Users/mac/Documents/workspace/spotahome/code/opensource/symfony/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHttpTransport.php:64  at Symfony\Component\Mailer\Bridge\Mailchimp\Transport\MandrillHttpTransport->doSendHttp(object(SentMessage))     (/Users/mac/Documents/workspace/spotahome/code/opensource/symfony/src/Symfony/Component/Mailer/Transport/AbstractHttpTransport.php:71)  at Symfony\Component\Mailer\Transport\AbstractHttpTransport->doSend(object(SentMessage))     (/Users/mac/Documents/workspace/spotahome/code/opensource/symfony/src/Symfony/Component/Mailer/Transport/AbstractTransport.php:71)  at Symfony\Component\Mailer\Transport\AbstractTransport->send(object(SentMessage), object(DelayedEnvelope))     (/Users/mac/Documents/workspace/spotahome/code/opensource/symfony/src/Symfony/Component/Mailer/Transport/Transports.php:51)  at Symfony\Component\Mailer\Transport\Transports->send(object(Email), null)     (/Users/mac/Documents/workspace/spotahome/code/opensource/symfony/src/Symfony/Component/Mailer/Mailer.php:41)  at Symfony\Component\Mailer\Mailer->send(object(Email))     (src/Controller/MailerController.php:20)  at App\Controller\MailerController->index(object(Mailer))```## The fix, how does it work:According to [documentation](https://mandrillapp.com/api/docs/messages.html), the endpoint `https://mandrillapp.com/api/1.0/messages/send-raw.json` returns an array of results in case of success:```[    {        "email": "recipient.email@example.com",        "status": "sent",        "reject_reason": "hard-bounce",        "_id": "abc123abc123abc123abc123abc123"    }]```in case of an error:```{    "status": "error",    "code": 12,    "name": "Unknown_Subaccount",    "message": "No subaccount exists with the id 'customer-123'"}```The fix just sets the right message id.Commits-------fa3f901 mailer: mailchimp bridge is throwing undefined index _id when setting message id in mandrill http transport