then(onfulfilled, onrejected) {
        return this.promiseFinished().then(value => onfulfilled ? Promise.resolve(onfulfilled(value)) : value, reason => onrejected ? Promise.resolve(onrejected(reason)) : Promise.reject(reason));
    }
Instead of awaiting the response status and trailers, you can
just as well await this call itself to receive the server outcome.
Note that it may still be valid to send more request messages.