xxxxx.xxxxx.campaign.budget.update
        ===================================
        更新一个推广计划的日限额

def update(self, campaign_id, budget, nick=None):
        '''xxxxx.xxxxx.campaign.budget.update
        ===================================
        更新一个推广计划的日限额'''
        request = TOPRequest('xxxxx.xxxxx.campaign.budget.update')
        request['campaign_id'] = campaign_id
        request['budget'] = budget
        if nick!=None: request['nick'] = nick
        self.create(self.execute(request), fields=['success','result','success','result_code','result_message'], models={'result':CampaignBudget})
        return self.result