This means that the model:
* Implements IConvertible so that Jint may implicitly convert
it to other types as if it were just a string.
* The Values & Values properties are perhaps a little less confusing
now, and present a more consistent set of state, regardless of
the inner model.
* There's a minor simplification to ToString, since String.Join already
did what we wanted for one or many values.
Notable though is - because this class implements IConvertible,
it is no longer CLS compliant. We do not currently mark this assembly
as CLS complient so as things stand that's OK. I do not know if there
are any plans/expectations for CLS compliance in Elsa.
If we do plan to mark the assemblies as compliant then we must mark this
class [CLSCompliant(false)].
It actually turns out that no change is required to the SignalReceived
activity in order to achieve this. As these unit tests prove & document,
that is already the current behaviour.
