Only set Auto-Submitted header once

The emails on push feature reuses the same email object, to avoid the expensive
work of generating the email body.

This interceptor would previously set multiple values for the same header, as
that's the mail gem's default behaviour when called with the same header more
than once.

We don't want to change the emails on push service (although it's the only place
where this happens), but fixing the interceptor is just as sensible anyway.

changelogs/unreleased/39593-emails-on-push-are-sent-to-only-the-first-recipient-when-using-aws-ses.yml

+---
+title: Only set Auto-Submitted header once for emails on push
+merge_request:
+author:
+type: fixed

lib/additional_email_headers_interceptor.rb

 class AdditionalEmailHeadersInterceptor
   def self.delivering_email(message)
-    message.headers(
-      'Auto-Submitted' => 'auto-generated',
-      'X-Auto-Response-Suppress' => 'All'
-    )
+    message.header['Auto-Submitted'] ||= 'auto-generated'
+    message.header['X-Auto-Response-Suppress'] ||= 'All'
   end
 end

spec/lib/additional_email_headers_interceptor_spec.rb

 require 'spec_helper'
 
 describe AdditionalEmailHeadersInterceptor do
-  it 'adds Auto-Submitted header' do
-    mail = ActionMailer::Base.mail(to: 'test@mail.com', from: 'info@mail.com', body: 'hello').deliver
+  let(:mail) do
+    ActionMailer::Base.mail(to: 'test@mail.com', from: 'info@mail.com', body: 'hello')
+  end
+
+  before do
+    mail.deliver_now
+  end
 
+  it 'adds Auto-Submitted header' do
     expect(mail.header['To'].value).to eq('test@mail.com')
     expect(mail.header['From'].value).to eq('info@mail.com')
     expect(mail.header['Auto-Submitted'].value).to eq('auto-generated')
     expect(mail.header['X-Auto-Response-Suppress'].value).to eq('All')
   end
+
+  context 'when the same mail object is sent twice' do
+    before do
+      mail.deliver_now
+    end
+
+    it 'does not add the Auto-Submitted header twice' do
+      expect(mail.header['Auto-Submitted'].value).to eq('auto-generated')
+      expect(mail.header['X-Auto-Response-Suppress'].value).to eq('All')
+    end
+  end
 end