Send Allow header when 405 Method Not Allowed

Required by RFC 2616

router.go

diff --git a/router.go b/router.go
index 8b5ff34..1f7bbe9 100644
--- a/router.go
+++ b/router.go
@@ -145,6 +145,8 @@
 	// Configurable http.Handler which is called when a request
 	// cannot be routed and HandleMethodNotAllowed is true.
 	// If it is not set, http.Error with http.StatusMethodNotAllowed is used.
+	// The "Allow" header with allowed request methods is set before the handler
+	// is called.
 	MethodNotAllowed http.Handler
 
 	// Function to handle panics recovered from http handlers.
@@ -333,6 +335,7 @@
 
 	// Handle 405
 	if r.HandleMethodNotAllowed {
+		var allow string
 		for method := range r.trees {
 			// Skip the requested method - we already tried this one
 			if method == req.Method {
@@ -341,17 +344,27 @@
 
 			handle, _, _ := r.trees[method].getValue(req.URL.Path)
 			if handle != nil {
-				if r.MethodNotAllowed != nil {
-					r.MethodNotAllowed.ServeHTTP(w, req)
+				// add request method to list of allowed methods
+				if len(allow) == 0 {
+					allow = method
 				} else {
-					http.Error(w,
-						http.StatusText(http.StatusMethodNotAllowed),
-						http.StatusMethodNotAllowed,
-					)
+					allow += ", " + method
 				}
-				return
 			}
 		}
+
+		if len(allow) > 0 {
+			w.Header().Set("Allow", allow)
+			if r.MethodNotAllowed != nil {
+				r.MethodNotAllowed.ServeHTTP(w, req)
+			} else {
+				http.Error(w,
+					http.StatusText(http.StatusMethodNotAllowed),
+					http.StatusMethodNotAllowed,
+				)
+			}
+			return
+		}
 	}
 
 	// Handle 404

router_test.go

diff --git a/router_test.go b/router_test.go
index e3141bd..0924d48 100644
--- a/router_test.go
+++ b/router_test.go
@@ -222,14 +222,30 @@
 	router := New()
 	router.POST("/path", handlerFunc)
 
-	// Test not allowed
+	// test not allowed
 	r, _ := http.NewRequest("GET", "/path", nil)
 	w := httptest.NewRecorder()
 	router.ServeHTTP(w, r)
 	if !(w.Code == http.StatusMethodNotAllowed) {
 		t.Errorf("NotAllowed handling failed: Code=%d, Header=%v", w.Code, w.Header())
+	} else if allow := w.Header().Get("Allow"); allow != "POST" {
+		t.Error("unexpected Allow header value: " + allow)
 	}
 
+	// add another method
+	router.DELETE("/path", handlerFunc)
+
+	// test again
+	r, _ = http.NewRequest("GET", "/path", nil)
+	w = httptest.NewRecorder()
+	router.ServeHTTP(w, r)
+	if !(w.Code == http.StatusMethodNotAllowed) {
+		t.Errorf("NotAllowed handling failed: Code=%d, Header=%v", w.Code, w.Header())
+	} else if allow := w.Header().Get("Allow"); allow != "POST, DELETE" && allow != "DELETE, POST" {
+		t.Error("unexpected Allow header value: " + allow)
+	}
+
+	// test custom handler
 	w = httptest.NewRecorder()
 	responseText := "custom method"
 	router.MethodNotAllowed = http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
@@ -243,6 +259,9 @@
 	if w.Code != http.StatusTeapot {
 		t.Errorf("unexpected response code %d want %d", w.Code, http.StatusTeapot)
 	}
+	if allow := w.Header().Get("Allow"); allow != "POST, DELETE" && allow != "DELETE, POST" {
+		t.Error("unexpected Allow header value: " + allow)
+	}
 }
 
 func TestRouterNotFound(t *testing.T) {