Fixes #323 to prevent serialization of token expiry

[bluesliverx]

This should fix #323 (as suggested by @jetersen) to prevent serialization of a field that cannot be serialized.
See also #268.

Testing done

I tested this out locally on a Jenkins instance and didn't see any problems with it, but as I never reproduced the original error, I could use some help from @omehegan to test this out.

Submitter checklist

Preview Give feedback

1. Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
2. Ensure that the pull request title represents the desired changelog entry
3. Please describe what you did
4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue (*I'm sure not sure how to test this exactly...)

[omehegan]

I didn't have this issue myself, it was a customer of mine that ran into it. I have let them know about the fix, hopefully they can do some testing to verify it.