Please answer these questions before submitting your issue. Thanks!
I wrote a proxy server and my colleagues found that the authentication information being forwarded was wrong. I compared Mozilla's documentation Authorization and found that the SetBasicAuth method was not in the form of an instance.
Example.

Authorization: Basic YWxhZGRpbjpvcGVuc2VzYW1l

But SetBasicAuth result is the following.

Authorization:[Basic c3NvX2FkbWluOjt4ODg4OA==]

The value of the Authorization in the example is a string, but what SetBasicAuth gets is a []string.

What version of Go are you using (go version)?

go version go1.11 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build572473428=/tmp/go-build -gno-record-gcc-switches"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

func (s *Server) ProxyServer(w http.ResponseWriter, r *http.Request) {
	director := func(req *http.Request) {
		req.SetBasicAuth(s.User, s.Pass)
		req.URL.Scheme = target.Scheme
		req.URL.Host = target.Host
		req.Host = target.Host
	}
	proxy := &httputil.ReverseProxy{Director: director}
	proxy.ServeHTTP(w, r)
}

What did you expect to see?

The value of Authorization is a string type.

Authorization: Basic YWxhZGRpbjpvcGVuc2VzYW1l

What did you see instead?

The value of Authorization is a []string type.

Authorization:[Basic c3NvX2FkbWluOjt4ODg4OA==]