Fix error on default filters page

[mzazrivec]

With #4116 in place, navigate to: My Settings → Default Filters, you'll witness the following error:

F, [2018-06-12T16:20:12.505321 #10918] FATAL -- : Error caught: [ArgumentError] comparison of Array with Array failed
manageiq-ui-classic/app/presenters/tree_builder_default_filters.rb:21:in `sort_by'
manageiq-ui-classic/app/presenters/tree_builder_default_filters.rb:21:in `prepare_data'
manageiq-ui-classic/app/presenters/tree_builder_default_filters.rb:30:in `initialize'
manageiq-ui-classic/app/controllers/configuration_controller.rb:511:in `new'
manageiq-ui-classic/app/controllers/configuration_controller.rb:511:in `set_form_vars'
manageiq-ui-classic/app/controllers/configuration_controller.rb:64:in `edit'
manageiq-ui-classic/app/controllers/configuration_controller.rb:72:in `change_tab'

[miq-bot]

Checked commit mzazrivec@23acdce with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 0 offenses detected
Everything looks fine. 🍰

[ZitaNemeckova]

Tested in UI:

LGTM 👍