Schulze with single canidate doesn't output a winner

[lune-stone]

Noticed this by chance in my own testing. While trivial to work around I figured I'd report it. I haven't checked if other methods have similar issues.

Here is a failing test:

    #[test]
    fn schulze_single_canidate() -> Result<(), TallyError> {

        let candidates = vec!["A".to_string()];

        let votes_raw = "A";

        let votes = Cursor::new(votes_raw);
        let votes = util::read_votes(votes).unwrap();

        // Margin
        let mut tally = DefaultSchulzeTally::with_candidates(1, Variant::Margin, candidates.clone());
        for (vote, weight) in votes.iter() {
            match vote {
                util::ParsedVote::Ranked(v) => tally.ranked_add_weighted(v, *weight)?,
                util::ParsedVote::Unranked(v) => tally.add_weighted(v, *weight)?,
            }
        }
        assert_eq!(tally.winners().into_unranked()[0], "A".to_string());

        // Winning
        let mut tally = DefaultSchulzeTally::with_candidates(1, Variant::Winning, candidates.clone());
        for (vote, weight) in votes.iter() {
            match vote {
                util::ParsedVote::Ranked(v) => tally.ranked_add_weighted(v, *weight)?,
                util::ParsedVote::Unranked(v) => tally.add_weighted(v, *weight)?,
            }
        }
        assert_eq!(tally.winners().into_unranked()[0], "A".to_string());

        // Ratio
        let votes = Cursor::new(votes_raw);
        let votes = util::read_votes(votes).unwrap(); // reparse votes as f64
        let mut tally = SchulzeTally::<_, f64>::with_candidates(1, Variant::Ratio, candidates.clone());
        for (vote, weight) in votes.iter() {
            match vote {
                util::ParsedVote::Ranked(v) => tally.ranked_add_weighted(v, *weight)?,
                util::ParsedVote::Unranked(v) => tally.add_weighted(v, *weight)?,
            }
        }
        assert_eq!(tally.winners().into_unranked()[0], "A".to_string());

        Ok(())
    }