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