Update epics index JSON response

For JSON requests epics index page returns a JSON array of epics. This
will be used on epic roadmap page (#3559). Pagination is disabled in
this case - it's not supported for the first iteration on roadmap page.

Until now JSON response for epics index action returned an HTML list of
epics but it was not used anywhere in our code.

Related #3559

ee/app/controllers/groups/epics_controller.rb

@@ -17,9 +17,7 @@ class Groups::EpicsController < Groups::ApplicationController
     respond_to do |format|
       format.html
       format.json do
-        render json: {
-          html: view_to_html_string("groups/epics/_epics")
-        }
+        render json: serializer.represent(@epics)
       end
     end
   end
@@ -38,6 +36,10 @@ class Groups::EpicsController < Groups::ApplicationController
 
   private
 
+  def pagination_disabled?
+    request.format.json?
+  end
+
   def epic
     @issuable = @epic ||= @group.epics.find_by(iid: params[:epic_id] || params[:id])
 

spec/ee/spec/controllers/groups/epics_controller_spec.rb

@@ -76,6 +76,39 @@ describe Groups::EpicsController do
           expect(response).to have_gitlab_http_status(200)
         end
       end
+
+      context 'when format is JSON' do
+        before do
+          allow(Kaminari.config).to receive(:default_per_page).and_return(1)
+        end
+
+        def list_epics
+          get :index, group_id: group, format: :json
+        end
+
+        it 'returns a list of epics' do
+          list_epics
+
+          expect(json_response).to be_an Array
+        end
+
+        it 'does not use pagination' do
+          list_epics
+
+          expect(json_response.size).to eq(2)
+        end
+
+        it 'returns correct epic attributes' do
+          list_epics
+          item = json_response.first
+          epic = Epic.find(item['id'])
+
+          expect(item['group_id']).to eq(group.id)
+          expect(item['start_date']).to eq(epic.start_date)
+          expect(item['end_date']).to eq(epic.end_date)
+          expect(item['web_url']).to eq(group_epic_path(group, epic))
+        end
+      end
     end
 
     describe 'GET #show' do