Clean Topic#slug when SiteSetting.slug_generation_method changes.

https://meta.discourse.org/t/removing-the-concept-of-slugs-for-some-languages/26643/24?u=tgxworld

spec/models/topic_spec.rb

@@ -201,6 +201,26 @@ describe Topic do
         end
       end
     end
+
+    describe 'when updating SiteSetting.slug_generation_method' do
+      let(:title) { '熱帶風暴畫眉熱帶風暴畫眉熱帶風暴畫眉' }
+      let(:slug) { 'topic' }
+
+      it 'should clear the slug' do
+        topic.save!
+        expect(topic.title).to eq(title)
+        expect(topic.slug).to eq(slug)
+
+        begin
+          Scheduler::Defer.async = false
+          SiteSetting.update(name: 'slug_generation_method', value: 'encoded')
+          expect(topic.reload.read_attribute(:slug)).to eq(nil)
+        ensure
+          SiteSetting.where(name: 'slug_generation_method').destroy_all
+          Scheduler::Defer.async = true
+        end
+      end
+    end
   end
 
   context "updating a title to be shorter" do