如何在golang中编写实用的https客户端

Question

我正在golang中构建一个https客户端，它应该连接到指定的主机并从主机的头部或证书中获取服务器名称。这是我的代码

package main

import (
    "crypto/tls"
    "crypto/x509"
    "fmt"
    "io/ioutil"
    "net/http"
)

func main() {
    caCert, _ := ioutil.ReadFile("./cacert.pem")
    caCertPool := x509.NewCertPool()
    caCertPool.AppendCertsFromPEM(caCert)

    conf := &tls.Config{RootCAs: caCertPool}
    conf.BuildNameToCertificate()
    tr := &http.Transport{TLSClientConfig: conf}
    cli := &http.Client{Transport: tr}
    resp, _ := cli.Head("https://www.google.com")
    fmt.Println(resp.TLS.ServerName)
}

但它正陷入恐慌

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x88 pc=0x401314]

goroutine 1 [running]:
panic(0x7145e0, 0xc82000a100)
/usr/local/go/src/runtime/panic.go:464 +0x3e6
main.main()
/home/huangjian/anti-GFW/go-checkgoogleip/checkip.go:22 +0x314
exit status 2

也许我缺乏对公钥加密的扎实了解，但现在我只想让它发挥作用。有谁能告诉我我在哪里做错了以及如何纠正它？