net/http: fix bug where http2 wasn't enabled on DefaultTransport

I had accidentally disabled a headline feature at the last second. :(

Fixes #14391

Change-Id: I1992c9b801072b7538b95c55242be174075ff932
Reviewed-on: https://go-review.googlesource.com/19672
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/22035
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transport.go

@@ -176,9 +176,13 @@ func (t *Transport) onceSetNextProtoDefaults() {
 		// Issue 14275.
 		return
 	}
-	if t.ExpectContinueTimeout != 0 {
-		// Unsupported in http2, so disable http2 for now.
-		// Issue 13851.
+	if t.ExpectContinueTimeout != 0 && t != DefaultTransport {
+		// ExpectContinueTimeout is unsupported in http2, so
+		// if they explicitly asked for it (as opposed to just
+		// using the DefaultTransport, which sets it), then
+		// disable http2 for now.
+		//
+		// Issue 13851. (and changed in Issue 14391)
 		return
 	}
 	t2, err := http2configureTransport(t)

src/net/http/transport_test.go

@@ -2888,6 +2888,11 @@ func TestTransportAutomaticHTTP2(t *testing.T) {
 	testTransportAutoHTTP(t, &Transport{}, true)
 }
 
+// golang.org/issue/14391: also check DefaultTransport
+func TestTransportAutomaticHTTP2_DefaultTransport(t *testing.T) {
+	testTransportAutoHTTP(t, DefaultTransport.(*Transport), true)
+}
+
 func TestTransportAutomaticHTTP2_TLSNextProto(t *testing.T) {
 	testTransportAutoHTTP(t, &Transport{
 		TLSNextProto: make(map[string]func(string, *tls.Conn) RoundTripper),