Rounding zero duration returns wrong result

[catamorphism]

The following test program:

use temporal_rs::{Calendar, Duration, TimeZone, ZonedDateTime};
use temporal_rs::options::{RelativeTo, RoundingOptions, Unit};

fn main() {
    let d0 = Duration::new(0, 0, 0, 0, 0, 0, 0, 0, 0, 0).unwrap();
    let mut rounding_options = RoundingOptions::default();
    rounding_options.largest_unit = Some(Unit::Day);
    rounding_options.smallest_unit = Some(Unit::Hour);
    let relative_to = ZonedDateTime::try_new(0, TimeZone::utc(), Calendar::default()).unwrap();
    let rounded_duration = d0.round(rounding_options, Some(RelativeTo::ZonedDateTime(relative_to))).unwrap();
    println!("{}", rounded_duration.to_string());
}

prints "-PT24H". It should print "PT0S".

I'll submit a PR shortly.

[catamorphism]

tc39/test262#4774 adds test262 coverage for this.