Defaults.keys() shouldn't return keys whose values are types (ie keys for which .has() will return False)
it should be possible to set a parameter that is defined through typing information only
A nonexistent inifile shouldn't interfere with the rest of the LayeredConfig object
make setting of undefined but typed values work with persistant sources
should be possible to set a setting on a subsection when the setting is only defined in root section
__iter__ didn't properly return all keys from parent objects when cascade was set
Even empty INIFiles (those initialized without a filename) should be able to handle a set() call