support Private Network Access spec (#127)

Based on [this Chrome
blog](https://developer.chrome.com/blog/private-network-access-preflight)
and the related W3C [draft
report](https://wicg.github.io/private-network-access).

cors.go

@@ -62,6 +62,9 @@ type Options struct {
 	// AllowCredentials indicates whether the request can include user credentials like
 	// cookies, HTTP authentication or client side SSL certificates.
 	AllowCredentials bool
+	// AllowPrivateNetwork indicates whether to accept cross-origin requests over a
+	// private network.
+	AllowPrivateNetwork bool
 	// OptionsPassthrough instructs preflight to let other potential next handlers to
 	// process the OPTIONS method. Turn this on if your application handles OPTIONS.
 	OptionsPassthrough bool
@@ -103,6 +106,7 @@ type Cors struct {
 	// Status code to use for successful OPTIONS requests
 	optionsSuccessStatus int
 	allowCredentials     bool
+	allowPrivateNetwork  bool
 	optionPassthrough    bool
 }
 
@@ -113,6 +117,7 @@ func New(options Options) *Cors {
 		allowOriginFunc:        options.AllowOriginFunc,
 		allowOriginRequestFunc: options.AllowOriginRequestFunc,
 		allowCredentials:       options.AllowCredentials,
+		allowPrivateNetwork:    options.AllowPrivateNetwork,
 		maxAge:                 options.MaxAge,
 		optionPassthrough:      options.OptionsPassthrough,
 	}
@@ -319,6 +324,9 @@ func (c *Cors) handlePreflight(w http.ResponseWriter, r *http.Request) {
 	if c.allowCredentials {
 		headers.Set("Access-Control-Allow-Credentials", "true")
 	}
+	if c.allowPrivateNetwork && r.Header.Get("Access-Control-Request-Private-Network") == "true" {
+		headers.Set("Access-Control-Allow-Private-Network", "true")
+	}
 	if c.maxAge > 0 {
 		headers.Set("Access-Control-Max-Age", strconv.Itoa(c.maxAge))
 	}

cors_test.go

@@ -18,6 +18,7 @@ var allHeaders = []string{
 	"Access-Control-Allow-Methods",
 	"Access-Control-Allow-Headers",
 	"Access-Control-Allow-Credentials",
+	"Access-Control-Allow-Private-Network",
 	"Access-Control-Max-Age",
 	"Access-Control-Expose-Headers",
 }
@@ -387,6 +388,44 @@ func TestSpec(t *testing.T) {
 			},
 			true,
 		},
+		{
+			"AllowedPrivateNetwork",
+			Options{
+				AllowedOrigins:      []string{"http://foobar.com"},
+				AllowPrivateNetwork: true,
+			},
+			"OPTIONS",
+			map[string]string{
+				"Origin":                                 "http://foobar.com",
+				"Access-Control-Request-Method":          "GET",
+				"Access-Control-Request-Private-Network": "true",
+			},
+			map[string]string{
+				"Vary":                                 "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
+				"Access-Control-Allow-Origin":          "http://foobar.com",
+				"Access-Control-Allow-Methods":         "GET",
+				"Access-Control-Allow-Private-Network": "true",
+			},
+			true,
+		},
+		{
+			"DisallowedPrivateNetwork",
+			Options{
+				AllowedOrigins: []string{"http://foobar.com"},
+			},
+			"OPTIONS",
+			map[string]string{
+				"Origin":                                "http://foobar.com",
+				"Access-Control-Request-Method":         "GET",
+				"Access-Control-Request-PrivateNetwork": "true",
+			},
+			map[string]string{
+				"Vary":                         "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
+				"Access-Control-Allow-Origin":  "http://foobar.com",
+				"Access-Control-Allow-Methods": "GET",
+			},
+			true,
+		},
 		{
 			"OptionPassthrough",
 			Options{