Fix `UnicodeEncodeError` exception when managed by `supervisord`

When using `celery` with `supervisord` in Fedora 19(Schrödinger's cat), the `startup_info` contains unicode characters and causes failure in startup:

`UnicodeEncodeError: 'ascii' codec can't encode character u'\xf6' in position 138: ordinal not in range(128)`

I think it has something to do with `supervisord`'s redirection of `stdout` to local files. Anyway, a simple wrap of `safe_str` will fix the problem.