Improve unicode compat with old python-zephyr

Old python-zephyr (anything before 8b756475cdb2 in 2011, which includes
scripts' 2009 version) automatically decodes messages as UTF-8. Decoding twice
is bad, so only decode if the zephyr body isn't already in unicode.  This fixes
a crash when chiron-on-scripts receives a zephyr containing Unicode.