Fix notify_only_default_branch check for Slack service

The notify_only_default_branch property is using boolean_accessor
this means we need to check it using a question methods.
Also add specs for disabling this option.

app/models/project_services/chat_notification_service.rb

@@ -150,7 +150,7 @@ class ChatNotificationService < Service
   def notify_for_ref?(data)
     return true if data[:object_attributes][:tag]
-    return true unless notify_only_default_branch
+    return true unless notify_only_default_branch?
     data[:object_attributes][:ref] == project.default_branch
   end

changelogs/unreleased/mrchrisw-fix-slack-notify.yml

+---
+title: Fix notify_only_default_branch check for Slack service
+merge_request:
+author:

spec/support/slack_mattermost_notifications_shared_examples.rb

@@ -328,7 +328,7 @@ RSpec.shared_examples 'slack or mattermost notifications' do
     context 'only notify for the default branch' do
       context 'when enabled' do
         let(:pipeline) do
-          create(:ci_pipeline, project: project, status: 'failed', ref: 'not-the-default-branch')
+          create(:ci_pipeline, :failed, project: project, ref: 'not-the-default-branch')
         end
         before do
@@ -342,6 +342,18 @@ RSpec.shared_examples 'slack or mattermost notifications' do
           expect(result).to be_falsy
         end
       end
+
+      context 'when disabled' do
+        let(:pipeline) do
+          create(:ci_pipeline, :failed, project: project, ref: 'not-the-default-branch')
+        end
+
+        before do
+          chat_service.notify_only_default_branch = false
+        end
+
+        it_behaves_like 'call Slack/Mattermost API'
+      end
     end
   end
 end