The "X-MJ-CUSTOMID" header is wrong when sending segment email.

[XRaccourci]

What type of report is this:

Q	A
Bug report?	🔴
Feature request?
Enhancement?

Description:

Adding the 'X-MJ-CUSTOMID' header is available when sending the email in the Mailjet specific code into EmailBundle/Swiftmailer.
The header in message Swift_Mime_Message is not rewritten for the following messages because it is already present in the first message sent. But the "X-MJ-CUSTOMID" shall be unique
The solution is simply to delete the header before rewriting the correct header.

To correct the problem =
Add this "$message->getHeaders()->remove('X-MJ-CUSTOMID');" in line 65 in file Mautic\EmailBundle\Swiftmailer\Transport\MailjetTransport. (obviously before the line adding the correct X-MJ-CUSTOMID)

If a bug:

Q	A
Mautic version	2.12.1
PHP version	5.6.30

Steps to reproduce:

1. Create a segment
2. Add two leads at least in this segment
3. Create a manual email and set the precedent segment
4. Add log in Swift_Transport_AbstractSmtpTransport in the end of send function. Log this => $message->getHeaders()->get('X-MJ-CUSTOMID')->toString() before return.
5. Send this email
6. View the log. X-MJ-CUSTOMID is the same for the two differents emails. It's normally different because X-MJ-CUSTOMID is defined like this "leadIdHash-email".

Log errors:

First email headers =
Message-ID: f25dfe7ce718347c82e775477ac11748@mautic.localhost
X-MJ-CUSTOMID: 5a534d0ab2e08-firstemail@gmail.com

Second email headers =
Message-ID: e284329bfe60c1fea2861f01799f0169@mautic.localhost
X-MJ-CUSTOMID: 5a534d0ab2e08-firstemail@gmail.com