[Rails5] Fix spec/models/application_setting_spec.rb

Note: add rails5 to a branch name to trigger rails5 specific CI jobs.

https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/67501930

  1) ApplicationSetting validations repository_storages is a String, not an Array should eq "default"
     Failure/Error: it { expect(setting.repository_storages_before_type_cast).to eq('default') }

       expected: "default"
            got: "--- default\n...\n"

       (compared using ==)

       Diff:
       @@ -1,2 +1,3 @@
       -default
       +--- default
       +...
     # ./spec/models/application_setting_spec.rb:113:in `block (4 levels) in <top (required)>'