http2: don't autodetect Content-Type when the response has an empty body

This change was originally made in https://golang.org/cl/46631, however,
that change was applited to net/http/h2_bundle.go instead of x/net/http2.

Updates golang/go#20784

Change-Id: I947fa4c19f3efc400856573768140bece28276a2
Reviewed-on: https://go-review.googlesource.com/80135
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/go19_test.go

@@ -46,7 +46,6 @@ func TestServerGracefulShutdown(t *testing.T) {
 	wanth := [][2]string{
 		{":status", "200"},
 		{"x-foo", "bar"},
-		{"content-type", "text/plain; charset=utf-8"},
 		{"content-length", "0"},
 	}
 	if !reflect.DeepEqual(goth, wanth) {

http2/server.go

@@ -2322,7 +2322,7 @@ func (rws *responseWriterState) writeChunk(p []byte) (n int, err error) {
 			clen = strconv.Itoa(len(p))
 		}
 		_, hasContentType := rws.snapHeader["Content-Type"]
-		if !hasContentType && bodyAllowedForStatus(rws.status) {
+		if !hasContentType && bodyAllowedForStatus(rws.status) && len(p) > 0 {
 			ctype = http.DetectContentType(p)
 		}
 		var date string

http2/server_test.go

@@ -1718,7 +1718,6 @@ func TestServer_Response_NoData_Header_FooBar(t *testing.T) {
 		wanth := [][2]string{
 			{":status", "200"},
 			{"foo-bar", "some-value"},
-			{"content-type", "text/plain; charset=utf-8"},
 			{"content-length", "0"},
 		}
 		if !reflect.DeepEqual(goth, wanth) {
@@ -2953,7 +2952,6 @@ func TestServerDoesntWriteInvalidHeaders(t *testing.T) {
 		wanth := [][2]string{
 			{":status", "200"},
 			{"ok1", "x"},
-			{"content-type", "text/plain; charset=utf-8"},
 			{"content-length", "0"},
 		}
 		if !reflect.DeepEqual(goth, wanth) {
@@ -3266,7 +3264,6 @@ func TestServerNoAutoContentLengthOnHead(t *testing.T) {
 	headers := st.decodeHeader(h.HeaderBlockFragment())
 	want := [][2]string{
 		{":status", "200"},
-		{"content-type", "text/plain; charset=utf-8"},
 	}
 	if !reflect.DeepEqual(headers, want) {
 		t.Errorf("Headers mismatch.\n got: %q\nwant: %q\n", headers, want)