def __sethead(self, access_token):
        """Set HTTP header."""
        self.access_token = access_token
        now = calendar.timegm(datetime.datetime.now().timetuple())
        self.expiration = now + 1800
        self.head = {"Authorization": "Bearer %s" % access_token,
                     "User-Agent": self.user_agent
                     }
Set HTTP header.