net/tshttpproxy: support basic auth when available (#1354)

This allows proxy URLs such as:

    http://azurediamond:hunter2@192.168.122.154:38274

to be used in order to dial out to control, logs or derp servers.

Signed-off-by: Christine Dodrill <xe@tailscale.com>

net/tshttpproxy/tshttpproxy_test.go

@@ -0,0 +1,49 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows
+
+package tshttpproxy
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestGetAuthHeaderNoResult(t *testing.T) {
+	const proxyURL = `http://127.0.0.1:38274`
+
+	u, err := url.Parse(proxyURL)
+	if err != nil {
+		t.Fatalf("can't parse %q: %v", proxyURL, err)
+	}
+
+	ahval, err := GetAuthHeader(u)
+	if err != nil {
+		t.Fatalf("can't get auth header value: %v", err)
+	}
+
+	if ahval != "" {
+		t.Fatalf("wanted auth header value to be empty, got: %q", ahval)
+	}
+}
+
+func TestGetAuthHeaderBasicAuth(t *testing.T) {
+	const proxyURL = `http://user:password@127.0.0.1:38274`
+	const expect = `Basic dXNlcjpwYXNzd29yZA==`
+
+	u, err := url.Parse(proxyURL)
+	if err != nil {
+		t.Fatalf("can't parse %q: %v", proxyURL, err)
+	}
+
+	ahval, err := GetAuthHeader(u)
+	if err != nil {
+		t.Fatalf("can't get auth header value: %v", err)
+	}
+
+	if ahval != expect {
+		t.Fatalf("wrong auth header value: want: %q, got: %q", expect, ahval)
+	}
+}