Adds missing label status
Adds spec testing default status unselection

Checks that a user cannot unselect the default property of a status and
that the user can select the default property of a non-default status.
Prevent default selection for default status
Adds changelog entry
Merge pull request #531 from opf/fix/unselect_default_work_package_status

[FIX] Unselect default work package status