Fix cached secrets when running tests

When running multiple specs that have to overwrite the rails secrets,
it can happen that the condition:
"@cached_rails_secrets != Rails.application.secrets"
is not met and unexpected secrets are returned.

We have found this case while experimenting with tests related to the
Tenant secrets. In one case, assigning cached rails secrets to nil resulted in
a failure to detect when the 'rails.application.secrets' had changed.

app/models/tenant.rb

@@ -102,7 +102,7 @@ class Tenant < ApplicationRecord
 
       if @cached_rails_secrets != Rails.application.secrets
         @secrets = {}
-        @cached_rails_secrets = nil
+        @cached_rails_secrets = Rails.application.secrets
       end
 
       @secrets[current_schema] ||= Rails.application.secrets.merge(