Allow 0 max-age

Fixes #152

cors.go

@@ -66,7 +66,10 @@ type Options struct {
 	// API specification
 	ExposedHeaders []string
 	// MaxAge indicates how long (in seconds) the results of a preflight request
-	// can be cached
+	// can be cached. Default value is 0, which stands for no
+	// Access-Control-Max-Age header to be sent back, resulting in browsers
+	// using their default value (5s by spec). If you need to force a 0 max-age,
+	// set `MaxAge` to a negative value (ie: -1).
 	MaxAge int
 	// AllowCredentials indicates whether the request can include user credentials like
 	// cookies, HTTP authentication or client side SSL certificates.
@@ -362,6 +365,8 @@ func (c *Cors) handlePreflight(w http.ResponseWriter, r *http.Request) {
 	}
 	if c.maxAge > 0 {
 		headers.Set("Access-Control-Max-Age", strconv.Itoa(c.maxAge))
+	} else if c.maxAge < 0 {
+		headers.Set("Access-Control-Max-Age", "0")
 	}
 	c.logf("  Preflight response headers: %v", headers)
 }

cors_test.go

@@ -242,6 +242,26 @@ func TestSpec(t *testing.T) {
 			},
 			true,
 		},
+		{
+			"MaxAgeNegative",
+			Options{
+				AllowedOrigins: []string{"http://example.com/"},
+				AllowedMethods: []string{"GET"},
+				MaxAge:         -1,
+			},
+			"OPTIONS",
+			map[string]string{
+				"Origin":                        "http://example.com/",
+				"Access-Control-Request-Method": "GET",
+			},
+			map[string]string{
+				"Vary":                         "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
+				"Access-Control-Allow-Origin":  "http://example.com/",
+				"Access-Control-Allow-Methods": "GET",
+				"Access-Control-Max-Age":       "0",
+			},
+			true,
+		},
 		{
 			"AllowedMethod",
 			Options{