QVariant.toJsonValue() should convert from number types

This bug is an update for QTBUG-52097.

On 5.9, QVariant::toJsonValue() works for Q... types but not for C number types (int ...).

The actual implementation in qvariant.cpp call qVariantToHelper:

QJsonValue QVariant::toJsonValue() const
{
    return qVariantToHelper<QJsonValue>(d, handlerManager);
}

but all the methods like toInt call qNumVariantToHelper instead :

int QVariant::toInt(bool *ok) const
{
    return qNumVariantToHelper<int>(d, handlerManager, ok, d.data.i);
}

 
There is thus two flavours of q...VariantToHelper !

As a workaround, we have to use QJsonValue::fromVariant() but the API could be more consistent.