Fix #628 by enabling opt-in to nullable scopes

[grantholle]

This tries to rectify the aftermath of #608 by allowing you to opt-in to allowing scope to be null.

To enable pre 1.0.1 functionality, call allowNullScope() when setting the scope.

Bouncer::scope()
    ->to(1)
    ->allowNullScope();

[grantholle]

I don't know if I solved the failing test the right way?