Coerce labels parameter in Epics API

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19914

ee/lib/api/epics.rb

@@ -29,7 +29,7 @@ module API
         optional :state, type: String, values: %w[opened closed all], default: 'all',
                          desc: 'Return opened, closed, or all epics'
         optional :author_id, type: Integer, desc: 'Return epics which are authored by the user with the given ID'
-        optional :labels, type: String, desc: 'Comma-separated list of label names'
+        optional :labels, type: Array[String], coerce_with: Validations::Types::LabelsList.coerce, desc: 'Comma-separated list of label names'
         use :pagination
       end
       get ':id/(-/)epics' do
@@ -60,7 +60,7 @@ module API
         optional :start_date_is_fixed, type: Boolean, desc: 'Indicates start date should be sourced from start_date_fixed field not the issue milestones'
         optional :end_date, as: :due_date_fixed, type: String, desc: 'The due date of an epic'
         optional :due_date_is_fixed, type: Boolean, desc: 'Indicates due date should be sourced from due_date_fixed field not the issue milestones'
-        optional :labels, type: String, desc: 'Comma-separated list of label names'
+        optional :labels, type: Array[String], coerce_with: Validations::Types::LabelsList.coerce, desc: 'Comma-separated list of label names'
       end
       post ':id/(-/)epics' do
         authorize_can_create!
@@ -84,7 +84,7 @@ module API
         optional :start_date_is_fixed, type: Boolean, desc: 'Indicates start date should be sourced from start_date_fixed field not the issue milestones'
         optional :end_date, as: :due_date_fixed, type: String, desc: 'The due date of an epic'
         optional :due_date_is_fixed, type: Boolean, desc: 'Indicates due date should be sourced from due_date_fixed field not the issue milestones'
-        optional :labels, type: String, desc: 'Comma-separated list of label names'
+        optional :labels, type: Array[String], coerce_with: Validations::Types::LabelsList.coerce, desc: 'Comma-separated list of label names'
         optional :state_event, type: String, values: %w[reopen close], desc: 'State event for an epic'
         at_least_one_of :title, :description, :start_date_fixed, :start_date_is_fixed, :due_date_fixed, :due_date_is_fixed, :labels, :state_event
       end