WikiDiff#to_html returns a string with ASCII encoding if
the WikiJournal content has been Zlib compressed because
Zlib::Inflate.inflate returns strings with ASCII encoding.
Forcing the encoding to be UTF8 fixes this bug.
Contributed by Moritz Breit.
git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@7927 e93f8b46-1217-0410-a6f0-8f06a7374b81