Function one_or_none 

pub fn one_or_none<'a, T>(
    values: impl Iterator<Item = &'a str>,
) -> Result<Option<T>, ParseError>
where
    T: FromStr,
    <T as FromStr>::Err: Error + Send + Sync + 'static,

Read exactly one or none from a headers iterator

This function does not perform comma splitting like read_many_from_str