Add tests for Site Settings import/export

- extracted out site settings rake task to a class
- added tests for import and export of site settings

spec/services/site_settings_spec.rb

@@ -0,0 +1,25 @@
+require 'rails_helper'
+
+describe SiteSettingsTask do
+
+  before do
+    Discourse::Application.load_tasks
+  end
+
+  describe 'export' do
+    it 'creates a hash of all site settings' do
+      h = SiteSettingsTask.export_to_hash
+      expect(h.count).to be > 0
+    end
+  end
+
+  describe 'import' do
+    it 'updates site settings' do
+      yml = "title: Test"
+      log, counts = SiteSettingsTask.import(yml)
+      expect(log[0]).to eq "Changed title FROM: Discourse TO: Test"
+      expect(counts[:updated]).to eq 1
+      expect(SiteSetting.title).to eq "Test"
+    end
+  end
+end