"\xC2\x80" of latin1(iso-8859-1) is valid sequence of UTF-8.
to_utf8() should not return string if it is UTF-8 valid_encoding.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4922 e93f8b46-1217-0410-a6f0-8f06a7374b81