FEATURE: Include image url in topic serializer

The `image_url` is already included when fetching a list of topics, and
this commit adds it to the individual topic serializer so that it is
available via the api.

See

https://meta.discourse.org/t/single-topic-api-endpoint-should-contain-image-url/131020

for more details.

app/serializers/topic_view_serializer.rb

@@ -39,7 +39,8 @@ class TopicViewSerializer < ApplicationSerializer
     :featured_link_root_domain,
     :pinned_globally,
     :pinned_at,
-    :pinned_until
+    :pinned_until,
+    :image_url
   )
 
   attributes(

spec/serializers/topic_view_serializer_spec.rb

@@ -45,6 +45,30 @@ describe TopicViewSerializer do
     end
   end
 
+  describe '#image_url' do
+    let(:image_url) { 'http://meta.discourse.org/images/welcome/discourse-edit-post-animated.gif' }
+
+    describe 'when a topic has an image' do
+      it 'should return the image url' do
+        topic.update!(image_url: image_url)
+
+        json = serialize_topic(topic, user)
+
+        expect(json[:image_url]).to eq(image_url)
+      end
+    end
+
+    describe 'when a topic does not contain an image' do
+      it 'should return a nil image url' do
+
+        json = serialize_topic(topic, user)
+
+        expect(json.has_key? :image_url).to eq(true)
+        expect(json[:image_url]).to eq(nil)
+      end
+    end
+  end
+
   describe '#suggested_topics' do
     let(:topic2) { Fabricate(:topic) }