Rename message_id to mail_uid

This avoids potentially conflicting types with integers and strings.

lib/gitlab/email/receiver.rb

@@ -26,7 +26,7 @@ module Gitlab
 
       def mail_metadata
         {
-          message_id: mail.message_id,
+          mail_uid: mail.message_id,
           from_address: mail.from,
           to_address: mail.to,
           mail_key: mail_key,

spec/lib/gitlab/email/receiver_spec.rb

@@ -24,7 +24,7 @@ RSpec.describe Gitlab::Email::Receiver do
 
       stub_incoming_email_setting(enabled: true, address: "incoming+%{key}@appmail.example.com")
 
-      expect(receiver.mail_metadata.keys).to match_array(%i(message_id from_address to_address mail_key references delivered_to envelope_to x_envelope_to))
+      expect(receiver.mail_metadata.keys).to match_array(%i(mail_uid from_address to_address mail_key references delivered_to envelope_to x_envelope_to))
     end
 
     context 'when in a Delivered-To header' do