Method reject

cpp/src/delivery.cpp:50–50

delivery::~delivery() = default;
void delivery::accept() { settle_delivery(pn_object(), ACCEPTED); }
void delivery::reject() { settle_delivery(pn_object(), REJECTED); }
void delivery::release() { settle_delivery(pn_object(), RELEASED); }
void delivery::modify() { settle_delivery(pn_object(), MODIFIED); }