Fixes #5532 problem with MailjetTransport header X-MJ-CUSTOMID

[XRaccourci]

Q	A
Bug fix?	#5532
New feature?
Automated tests included?
Related user documentation PR URL
Related developer documentation PR URL
Issues addressed (#s or URLs)
BC breaks?
Deprecations?

Description:

Fixes issue #5532
Delete the header 'X-MJ-CUSTOMID' before rewriting the correct header in MailjetTransport.

Steps to reproduce the bug:

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". After fixes, the header is correct.

Steps to test this PR:

1. Apply PR
2. Repeat Steps 1-6 above
3. Notice value is correct

List deprecations along with the new alternative:

List backwards compatibility breaks:

[dbhurley]

+1

[escopecz]

👍