Fix client certs

[mrinnetmaki]

Or at least attempt to. The challenge is that the error we get only manifests in production environment.

Risk analysis

• Risks of the implementation in this pull request have been analyzed with following results:
• No significant risks.

Significance

• Analysis, is the software change considered significant (ref. MDCG 2020-3, chart C), has been made with following result:
• Not significant change, can be made to the product certified according MDD.
  • Reasons for the decision - No changes to product, only to security config

Security check-up

https://github.com/OWASP/www-project-top-ten/blob/master/index.md

• A01:2021-Broken Access Control - Improves overall security config by using the client certificate feature in the way that's documented in the library.
• A02:2021-Cryptographic Failures - No direct changes to crypto or sensitive data, but an improvement in PKI management.
• A03:2021-Injection - No changes in data input or database code
• A04:2021-Insecure Design - Not directly a change in design, but changing the implementation, to be more robust.
• A05:2021-Security Misconfiguration - Improves overall security config by using the client certificate feature in the way that's documented in the library.
• A06:2021-Vulnerable and Outdated Components - The original error causing this was due to an outdated component. This PR fixes the issue.
• A07:2021-Identification and Authentication Failures - Improving the way our app is identified by Omatietovaranto.
• A08:2021-Software and Data Integrity Failures - No changes to artifacts or build process.
• A09:2021-Security Logging and Monitoring Failures - No changes to logging, no new needs identified.
• A10:2021-Server-Side Request Forgery - No changes to URL or link processing or request handling.

[mrinnetmaki]

Superseded by #242. That's the real fix.