FIX: replace default welcome topic post with new value from wizard

Previously the text entered in the wizard would be prepended onto the
default first paragraph.

spec/lib/introduction_updater_spec.rb

@@ -55,4 +55,42 @@ describe IntroductionUpdater do
       end
     end
   end
+
+  describe "update_summary" do
+    let(:welcome_topic) do
+      topic = Fabricate(:topic, title: I18n.t("discourse_welcome_topic.title"))
+      Fabricate(
+        :post,
+        topic: topic,
+        raw: I18n.t("discourse_welcome_topic.body", base_path: Discourse.base_path),
+        post_number: 1
+      )
+      topic
+    end
+
+    let(:first_post) { welcome_topic.posts.first }
+
+    let(:new_summary) { "Welcome to my new site. It's gonna be good." }
+
+    subject { IntroductionUpdater.new(Fabricate(:admin)).update_summary(new_summary) }
+
+    before do
+      SiteSetting.welcome_topic_id = welcome_topic.id
+    end
+
+    it "completely replaces post if it has default value" do
+      subject
+      expect {
+        expect(first_post.reload.raw).to eq(new_summary)
+      }.to_not change { welcome_topic.reload.category_id }
+    end
+
+    it "only replaces first paragraph if it has custom content" do
+      paragraph1 = "This is the summary of my community"
+      paragraph2 = "And this is something I added later"
+      first_post.update!(raw: [paragraph1, paragraph2].join("\n\n"))
+      subject
+      expect(first_post.reload.raw).to eq([new_summary, paragraph2].join("\n\n"))
+    end
+  end
 end