Allow parameter-less element factory when ofType<void> is specified

[mischkl]

When specifying an element with ofType<void>, it was my expectation that I could then use the element factory without specifying an argument, e.g. AuthActions.LOGOUT(). Instead it turns out I have to do AuthActions.LOGOUT(null) or AuthActions.LOGOUT(undefined), which feels rather unnecessary. It seems to me like there should be a way to enable this behavior, by having two different types for the factory depending on whether the type is void or not.

[pelotom]

There was an abortive attempt at this with #15 but we are definitely going to get this in for version 2, thanks for the issue!