采用ATS的默认配置，httpdns 在iOS9平台上报错，在iOS10及以上系统没有问题

httpdns_ios_demo[272:26192] error: Error Domain=NSURLErrorDomain Code=-1022 "The resource could not be loaded because the App Transport Security policy requires the use of a secure connection." UserInfo={NSErrorFailingURLStringKey=http://210.192.117.229/, NSErrorFailingURLKey=http://210.192.117.229/, NSLocalizedDescription=The resource could not be loaded because the App Transport Security policy requires the use of a secure connection., NSUnderlyingError=0x165e36e0 {Error Domain=kCFErrorDomainCFNetwork Code=-1022 "(null)"}}