net/http: don't send Accept-Encoding on HEAD requests

Works around a bug in nginx: http://trac.nginx.org/nginx/ticket/358

Fixes #5522

R=iant
CC=gobot, golang-dev
https://golang.org/cl/9627043

src/pkg/net/http/transport.go

@@ -831,10 +831,15 @@ func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err err
 	// uncompress the gzip stream if we were the layer that
 	// requested it.
 	requestedGzip := false
-	if !pc.t.DisableCompression && req.Header.Get("Accept-Encoding") == "" {
+	if !pc.t.DisableCompression && req.Header.Get("Accept-Encoding") == "" && req.Method != "HEAD" {
 		// Request gzip only, not deflate. Deflate is ambiguous and
 		// not as universally supported anyway.
 		// See: http://www.gzip.org/zlib/zlib_faq.html#faq38
+		//
+		// Note that we don't request this for HEAD requests,
+		// due to a bug in nginx:
+		//   http://trac.nginx.org/nginx/ticket/358
+		//   http://golang.org/issue/5522
 		requestedGzip = true
 		req.extraHeaders().Set("Accept-Encoding", "gzip")
 	}

src/pkg/net/http/transport_test.go

@@ -585,13 +585,16 @@ func TestTransportGzip(t *testing.T) {
 	const testString = "The test string aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
 	const nRandBytes = 1024 * 1024
 	ts := httptest.NewServer(HandlerFunc(func(rw ResponseWriter, req *Request) {
+		if req.Method == "HEAD" {
+			if g := req.Header.Get("Accept-Encoding"); g != "" {
+				t.Errorf("HEAD request sent with Accept-Encoding of %q; want none", g)
+			}
+			return
+		}
 		if g, e := req.Header.Get("Accept-Encoding"), "gzip"; g != e {
 			t.Errorf("Accept-Encoding = %q, want %q", g, e)
 		}
 		rw.Header().Set("Content-Encoding", "gzip")
-		if req.Method == "HEAD" {
-			return
-		}
 
 		var w io.Writer = rw
 		var buf bytes.Buffer