comments: fixup "show hidden comments" link.

Fixup for 896f144eb.

app/controllers/comments_controller.rb

@@ -5,7 +5,7 @@ class CommentsController < ApplicationController
   skip_before_action :api_check
 
   def index
-    params[:group_by] ||= "comment" if params[:search]
+    params[:group_by] ||= "comment" if params[:search].present?
 
     if params[:group_by] == "comment" || request.format == Mime::Type.lookup("application/atom+xml")
       index_by_comment

test/functional/comments_controller_test.rb

@@ -21,8 +21,10 @@ class CommentsControllerTest < ActionDispatch::IntegrationTest
     context "index action" do
       context "grouped by post" do
         should "render all comments for .js" do
-          get comments_path(post_id: @post.id, group_by: "post", format: "js"), xhr: true
+          get comments_path(post_id: @post.id), xhr: true
+
           assert_response :success
+          assert_equal("text/javascript", response.media_type)
         end
 
         should "show posts with visible comments" do