Datetime rounds fractional seconds instead of truncating

[alanhdu]

According to the TOML spec:

The precision of fractional seconds is implementation specific, but at least millisecond precision is expected. If the value contains greater precision than the implementation can support, the additional precision must be truncated, not rounded.

Right now, toml seems to round fractional seconds instead. One fun consequence of this is that:

extern crate toml;
fn main() {
    let s = "n = 17:45:00.999999999999999999";
    let data: toml::Value = toml::from_str(s).unwrap();
    let s1 = toml::to_string(&data).unwrap();
    println!("{:?}", s1);
}

prints out 17:45:001, which isn't a valid time.

Found with https://github.com/rust-fuzz/targets