static <T extends Response> RetryRuleWithContent<T> onUnprocessed() {
        return onUnprocessed(Backoff.ofDefault());
    }
Returns a {@link RetryRuleWithContent} that retries with the
{@linkplain Backoff#ofDefault() default backoff} on an {@link UnprocessedRequestException} which means
that the request has not been processed by the server. Therefore, you can safely retry the request
without worrying about the idempotency of the request.
core/src/main/java/com/linecorp/armeria/client/retry/RetryRuleWithContent.java