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

diff --git a/src/pkg/http/serve_test.go b/src/pkg/http/serve_test.go
index dc4594a..40de547 100644
--- a/src/pkg/http/serve_test.go
+++ b/src/pkg/http/serve_test.go
@@ -781,6 +781,21 @@
 	}
 }
 
+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

diff --git a/src/pkg/http/server.go b/src/pkg/http/server.go
index bb47430..7f1b8a2 100644
--- a/src/pkg/http/server.go
+++ b/src/pkg/http/server.go
@@ -254,7 +254,7 @@
 		}
 	}
 
-	if w.header.Get("Date") == "" {
+	if _, ok := w.header["Date"]; !ok {
 		w.Header().Set("Date", time.UTC().Format(TimeFormat))
 	}