Fix: Private properties of descendands handling more properly in form "ClassName::propertyName"

Before this fix private properties of descendants collected as is
with NUL character: "<NUL>ParentClass<NUL>propertyName". Such names may
be rendered as empty string somewhere. These NULs are just internals
of PHP.

Now collect such inherited properties as "ParentClass::propertyName".
Add debug case for serialize sources