Mailjet mailer now forwards headers from original email removes forbidden headers, and sends them to Maijlet Api

Additionally (seehttps://symfony.com/releases):

• 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.
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