Fix SMIME signed email sending, do not overwrite Content-Disposition

If signed email sending was enabled the Content-Disposition was set
but empty. This could cause issues with SMTP servers, in particular
sending via Amazon SES did not work at all as they expect a non-empty
Content-Disposition header. Response from SES with empty header was:
  554 Transaction failed: Expected disposition,
  got null (in reply to end of DATA command))

With this fix the Content-Disposition field is no longer overwritten
within the smime_signature_interceptor hook and the mails could be
delivered properly.

Changelog: fixed

lib/gitlab/email/hook/smime_signature_interceptor.rb

@@ -45,7 +45,6 @@ module Gitlab
           end
 
           def overwrite_headers(message, signed_email)
-            message.content_disposition = signed_email.content_disposition
             message.content_transfer_encoding = signed_email.content_transfer_encoding
             message.content_type = signed_email.content_type
           end

spec/lib/gitlab/email/hook/smime_signature_interceptor_spec.rb

@@ -50,6 +50,7 @@ RSpec.describe Gitlab::Email::Hook::SmimeSignatureInterceptor do
     expect(mail.header['To'].value).to eq('test@example.com')
     expect(mail.header['From'].value).to eq('info@example.com')
     expect(mail.header['Content-Type'].value).to match('multipart/signed').and match('protocol="application/x-pkcs7-signature"')
+    expect(mail.header.include?('Content-Disposition')).to eq(false)
 
     # verify signature and obtain pkcs7 encoded content
     p7enc = Gitlab::Email::Smime::Signer.verify_signature(