Variant::as<>() should not consume the variant.

I'm seeing the following errors:

void core::dbus::Property<T>::handle_changed(const core::dbus::types::Variant&) [with PropertyType = test::Service::Properties::ReadOnly]: Mismatch between expected and actual type reported by iterator:
         Expected: uint32
         Actual: invalid
void core::dbus::Property<T>::handle_changed(const core::dbus::types::Variant&) [with PropertyType = test::Service::Properties::ReadOnly]: Mismatch between expected and actual type reported by iterator:
         Expected: uint32
         Actual: invalid

This seems like Variant is consumed by the first caller of Variant::as<>(). This will be a problem for example if there are multiple handlers of a signal that passes an Variant as an argument.

There should not be a limit how many times Variant::as<>() or any other conversion function can be called.