Issues api - allow empty due_date arg

lib/api/issues.rb

@@ -55,7 +55,7 @@ module API
                  desc: 'Return issues ordered by `created_at` or `updated_at` fields.'
         optional :sort, type: String, values: %w[asc desc], default: 'desc',
                  desc: 'Return issues sorted in `asc` or `desc` order.'
-        optional :due_date, type: String, values: %w[0 overdue week month next_month_and_previous_two_weeks],
+        optional :due_date, type: String, values: %w[0 overdue week month next_month_and_previous_two_weeks] << '',
                  desc: 'Return issues that have no due date (`0`), or whose due date is this week, this month, between two weeks ago and next month, or which are overdue. Accepts: `overdue`, `week`, `month`, `next_month_and_previous_two_weeks`, `0`'
 
         use :issues_stats_params

spec/requests/api/issues/issues_spec.rb

@@ -404,6 +404,12 @@ RSpec.describe API::Issues do
           travel_back
         end
 
+        it 'returns them all when argument is empty' do
+          get api('/issues?due_date=', user)
+
+          expect_paginated_array_response(issue5.id, issue4.id, issue3.id, issue2.id, issue.id, closed_issue.id)
+        end
+
         it 'returns issues without due date' do
           get api('/issues?due_date=0', user)