python tornado facebook auth.py _signature() bug

unicode should be encoded before parts not body.

<code>

def _signature(self, args):
        parts = ["%s=%s" % (n, args[n].encode("utf-8") if isinstance(args[n], unicode) else args[n]) for n in sorted(args.keys())]
        body = "".join(parts) + self.settings["facebook_secret"]
        return hashlib.md5(body).hexdigest()

</code>