Add failing notification_recipient_service spec

spec/services/notification_recipient_service_spec.rb

+require 'spec_helper'
+
+describe NotificationRecipientService do
+  describe 'build_recipients' do
+    it 'due_date' do
+      # These folks are being filtered out because they can't receive notifications
+      # notification_recipient.rb#85
+      user = create(:user)
+      assignee = create(:user)
+      issue = create(:issue, :opened, due_date: Date.today, author: user, assignees: [assignee])
+
+      recipients = described_class.build_recipients(
+        issue,
+        issue.author,
+        action: "due_date",
+        skip_current_user: false
+      )
+
+      expect(recipients).to match_array([user, assignee])
+    end
+  end
+end