Updated system certificates are stored in plain text when they should be base64-encoded

[mj84]

Describe the bug
It seems I introduced a bug with #210 which causes updated certificates to be stored in plain text instead of base64-encoded.

To Reproduce
Update an already existing certificate, certificate manager will no longer show certificate's data such as common name, expiry date, etc.

Expected behavior
Updating an already existing certificate should not break the cert

Affected Endpoints:

• URL: /api/v1/system/certificate