SignalSpy qml type throws error when onX slot is present in cpp type

If a cpp type is exposed to qml via qmlRegisterType,
and this cpp type contains a signal `x` and a slot `onX`,
when a `SignalSpy` qml element is connected to signal `x`, an error is thrown:

`Error: Function.prototype.connect: this object is not a signal`

This makes it impossible to have interchangeable cpp and qml types with the same API based on the x/onX idiom.

example to reproduce:

https://github.com/marco-piccolino/qmltest