Fix visit of optional<T>
Fix visit of optional<T>

actor-framework/actor-framework,nq-ebaratte/actor-framework,nq-ebaratte/actor-framework,actor-framework/actor-framework,DavadDi/actor-framework,nq-ebaratte/actor-framework,DavadDi/actor-framework,actor-framework/actor-framework,nq-ebaratte/actor-framework,DavadDi/actor-framework,actor-framework/actor-framework,DavadDi/actor-framework