FIX: Sort group owners and members together (#15708)

Sorting group members worked always kept the group owners at the top of
the list. This commit keeps the group owners at the top of the list only
when no order exists.

spec/requests/groups_controller_spec.rb

@@ -547,6 +547,23 @@ describe GroupsController do
 
       expect(members.last['added_at']).to eq(first_user.created_at.as_json)
     end
+
+    it "can sort items" do
+      sign_in(user)
+      group.update!(visibility_level: Group.visibility_levels[:logged_on_users])
+      other_user = Fabricate(:user)
+      group.add_owner(other_user)
+
+      get "/groups/#{group.name}/members.json"
+
+      expect(response.parsed_body["members"].map { |u| u["id"] }).to eq([other_user.id, user.id])
+      expect(response.parsed_body["owners"].map { |u| u["id"] }).to eq([other_user.id])
+
+      get "/groups/#{group.name}/members.json?order=added_at&asc=1"
+
+      expect(response.parsed_body["members"].map { |u| u["id"] }).to eq([user.id, other_user.id])
+      expect(response.parsed_body["owners"].map { |u| u["id"] }).to eq([other_user.id])
+    end
   end
 
   describe '#posts_feed' do