don't double-decode utf8 strings in ::EncodeGuess

 * Recent versions of Text::CSV_{XS,PP} now automatically decode utf-8
   strings.  ::EncodeGuess wasn't paying attention to this and
   attempting to doubly-decode utf-8 strings, causing the fatal error:
   "Cannot decode string with wide character".  This patch skips the
   decode when the utf8 encoder is guessed and the automatic_UTF8 flag
   is set.

 * add utf8 test to t/tests/05_guess.t