http: permit handlers to explicitly remove the Date header

We'll do the right thing by default, but people wanting minimal
response sizes can explicitly remove the Date header.
(empty fields aren't written out)

R=rsc
CC=golang-dev
https://golang.org/cl/4634048

src/pkg/http/serve_test.go

@@ -781,6 +781,21 @@ func TestHandlerPanic(t *testing.T) {
 	}
 }
 
+func TestNoDate(t *testing.T) {
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header()["Date"] = nil
+	}))
+	defer ts.Close()
+	res, err := Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, present := res.Header["Date"]
+	if present {
+		t.Fatalf("Expected no Date header; got %v", res.Header["Date"])
+	}
+}
+
 type errorListener struct {
 	errs []os.Error
 }

src/pkg/http/server.go

@@ -254,7 +254,7 @@ func (w *response) WriteHeader(code int) {
 		}
 	}
 
-	if w.header.Get("Date") == "" {
+	if _, ok := w.header["Date"]; !ok {
 		w.Header().Set("Date", time.UTC().Format(TimeFormat))
 	}