Fix IEEE preview does not display month

[DevSiroukane]

Fix issue #3239 on master branch

On month change from comboBox on OptionalFieldsTab:

1. Month was not appear on IEEE style.
2. Month was not written correctly on SourceTab.
   both issues get fixed.

---

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[tobiasdiez]

This fix is now included in the maintable-beta branch and I think there is no urgency to also include it in the latest master. Thus I'll close this PR now.