Fix a delimiter detection problem in sniffer. Sniffing "a|b|c\r\n" was

Fix a delimiter detection problem in sniffer.  Sniffing "a|b|c\r\n" was
returning 'a' as the delimiter.  It now returns '|', but not because I
understood better what the code was supposed to do.  Would someone that
understands the idea behind _guess_delimiter() (see its doc string) look to
see if my fallback choice is better than before or if it's just serendipity
that I picked the proper delimiter?