set html encoding utf8 at Diff class (#12641)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11547 e93f8b46-1217-0410-a6f0-8f06a7374b81
fix that diff outputs become ??? in some non ASCII words (#12641)

Contributed by Filou Centrinov.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11549 e93f8b46-1217-0410-a6f0-8f06a7374b81