Move end date calculation to ActivePeriod class

- Minor spec tidy up

ee/app/models/incident_management/oncall_rotation.rb

@@ -11,7 +11,9 @@ module IncidentManagement
         end_time > start_time if present?
       end
 
-      def for_dates(start_date, end_date)
+      def for_date(start_date)
+        end_date = end_after_start? ? start_date : start_date.next_day
+
         [
           start_date.change(hour: start_time.hour, min: start_time.min),
           end_date.change(hour: end_time.hour, min: end_time.min)

ee/lib/incident_management/oncall_shift_generator.rb

@@ -129,10 +129,8 @@ module IncidentManagement
           #     expected_shift_count = 14          -> pretend it's a 2-week rotation
           #     shift_count = 2                    -> we're calculating the shift for the 3rd day
           # starts_at = Monday 00:00:00 + 8.hours + 2.days => Thursday 08:00:00
-          start_date = end_date = shift_cycle_starts_at + shift_count.days
-          end_date = start_date + 1.day unless rotation.active_period.end_after_start?
-
-          starts_at, ends_at = rotation.active_period.for_dates(start_date, end_date)
+          start_date = shift_cycle_starts_at + shift_count.days
+          starts_at, ends_at = rotation.active_period.for_date(start_date)
 
           shift_for(participant, [rotation.starts_at, starts_at].max, limit_end_time(ends_at))
         end

ee/spec/requests/api/issues_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe API::Issues, :mailer do
 
   let_it_be(:group) { create(:group) }
   let_it_be(:epic) { create(:epic, group: group) }
-  let_it_be(:group_project) { create( :project, :public, creator_id: user.id, namespace: group) }
+  let_it_be(:group_project) { create(:project, :public, creator_id: user.id, namespace: group) }
 
   let(:user2)             { create(:user) }
 

ee/spec/support/helpers/oncall_helpers.rb

@@ -4,6 +4,6 @@ module OncallHelpers
   def active_period_for_date_with_tz(date, rotation)
     date = date.in_time_zone(rotation.schedule.timezone)
 
-    rotation.active_period.for_dates(date, date)
+    rotation.active_period.for_date(date)
   end
 end