support non-preemptive authentication (#1888)

models/plugin/client/http_proxy.go

@@ -231,6 +231,7 @@ func removeProxyHeaders(req *http.Request) {
 func getBadResponse() *http.Response {
 	header := make(map[string][]string)
 	header["Proxy-Authenticate"] = []string{"Basic"}
+	header["Connection"] = []string{"close"}
 	res := &http.Response{
 		Status:     "407 Not authorized",
 		StatusCode: 407,