Evaluate default attribute procs on #execute

[hadronzoo]

Defaults attributes are evaluated on class definition, even if the default is a proc or a lambda. It would be useful and less surprising if default procs were executed when run! was called rather than when the interaction was required.

[AaronLasseigne]

This is certainly worth some thought.

[tfausak]

Yeah, I agree. We didn't consider this when we made #141. The offending line is in ActiveInteraction::Base. It has been like that for a long while. I think it originally came from 6b165d5.