[HttpConfigurationSystem] Proxy to previous system if section is empty

If the new HttpConfigurationSystem was put in place, there was no way to
access the old system for accessing non-System.Web settings, even though
the ChangeConfigurationSystem method was providing it as a return value.

Proxying to previous system fixes bxc#11972:
https://bugzilla.xamarin.com/show_bug.cgi?id=11972

(A unit test is added as well.)