fix: Use field translation in Jira.issue method

[Wojtini]

Fields param in issue are untranslated so if you use custom_field's name instead of id Jira API won't return it. Those changes are supposed to fix that.

Changes:

1. Extracted translation part to seperate methods
2. Changed variable names to be less abraviated
3. Reused those methods in Issue method
4. Missing tests for limiting fields on Issue method
5. Seperate tests for translation (Added duedate field to test issue)

To Do: Squash commits before merge