* Stop testing Encode::is_utf8 in decode_form_utf8: That doesn't work.

* decode_form_utf8 only fixed the utf-8 encoding for fields that were
  registered at the time it was called, which was before the
  formbuilder_setup hook. Fields added by the hook didn't get decoded.
  But it can't be put after the hook either, since plugins using the hook
  need to be able to use form values. To fix this dilemma, it's been changed
  to a decode_cgi_utf8, which is called on the cgi query object, before the
  form is set up, and decodes *all* cgi parameters.