Fix milestone semantic in Issue Search (#35)

If milestone was not defined in the rule, then only issues with milestones would be searched.

The semantic is now:

- No milestone set, consider issues without or without milestones
- Milestone is set consider only issues with the given milestone
- Milestone is * consider only issues with a milestone (any)
- Milestone is NONE consider only issues with no milestones set

Code was refactored and unit tests were added

fix typo