Fixed Preview theme UnicodeDecodeError when Theme Title is Non-ASCII

https://github.com/plone/plone.app.theming/commit/571a3f4a468b0655984b1de25b1ec900f9ceeb9a Fixed @@theming-controlpanel view UnicodeDecodeError

https://github.com/jianaijun/plone.app.theming/commit/9cb86ea091f86d8ba5f80c52e5de00a85c1ffc54 Fixed Preview theme (@@theming-controlpanel-mapper-getframe view)UnicodeDecodeError