Fix JSON Schema that validates data returned by board issues endpoint

spec/controllers/projects/boards/issues_controller_spec.rb

@@ -21,7 +21,7 @@ describe Projects::Boards::IssuesController do
       it 'returns issues that have the list label applied' do
         johndoe = create(:user, avatar: fixture_file_upload(File.join(Rails.root, 'spec/fixtures/dk.png')))
         create(:labeled_issue, project: project, labels: [planning])
-        create(:labeled_issue, project: project, labels: [development])
+        create(:labeled_issue, project: project, labels: [development], due_date: Date.tomorrow)
         create(:labeled_issue, project: project, labels: [development], assignee: johndoe)
         list_issues user: user, list_id: list2

spec/fixtures/api/schemas/issue.json

@@ -9,6 +9,7 @@
     "iid": { "type": "integer" },
     "title": { "type": "string" },
     "confidential": { "type": "boolean" },
+    "due_date": { "type": ["date", "null"] },
     "labels": {
       "type": "array",
       "items": {