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

What does this MR do and why?

This MR removes content_disposition overwrite within smime_signature_interceptor hook. Otherwise using Amazon SES does not work when signed email sending is enabled.

Testing this using rails console shows that the Content-Disposition header is empty:

irb(main):001:0> Notify.test_email('info@example.com', 'Message Subject', 'Message Body').deliver_now
Delivered mail 61af4dd24b27a_705a65a50164f0@example.com.mail (104.2ms)

=> #<Mail::Message:154760,Multipart: true, Headers: <Date: Tue, 07 Dec 2021 12:04:34 +0000>, <From: "GitLab" <gitlab@example.com>>, <Reply-To: "GitLab" <noreply@example.com>>, <To: info@example.com>, <Message-ID: <61af4dd24b27a_705a65a50164f0@example.com.mail>>, <Subject: Message Subject>, <Mime-Version: 1.0>, <Content-Type: multipart/signed; boundary=----E6CF76AC5B35F2D6A6BAD9702E480AA4; micalg=sha-256; protocol="application/x-pkcs7-signature">, <Content-Transfer-Encoding: 7bit>, <Content-Disposition: >, <Auto-Submitted: auto-generated>, <X-Auto-Response-Suppress: All>>

Response from SES:

554 Transaction failed: Expected disposition, got null (in reply to end of DATA command))

With this patch there is no empty Content-Disposition header anymore and SES accepts the mail:

irb(main):001:0> Notify.test_email('info@example.com', 'Message Subject', 'Message Body').deliver_now
Delivered mail 61af5a1272caf_d0a5a504255c@example.com.mail (72.7ms)
=> #<Mail::Message:154760, Multipart: true, Headers: <Date: Tue, 07 Dec 2021 12:56:50 +0000>, <From: "GitLab" <gitlab@example.com>>, <Reply-To: "GitLab" <noreply@example.com>>, <To: info@example.com>, <Message-ID: <61af5a1272caf_d0a5a504255c@example.com.mail>>, <Subject: Message Subject>, <Mime-Version: 1.0>, <Content-Type: multipart/signed; boundary=----BE07D25544FBCFE168AE754FC6702C70; micalg=sha-256; protocol="application/x-pkcs7-signature">, <Content-Transfer-Encoding: 7bit>, <Auto-Submitted: auto-generated>, <X-Auto-Response-Suppress: All>>

🛠 with ❤ at Siemens

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.