net/http: update bundled http2

Updates x/net/http2 to git rev 0c607074 for https://golang.org/cl/23311,
"http2: prevent Server from sending status 100 header after anything else"

New test is in the x/net/http2 package (not bundled to std).

Fixes #14030

Change-Id: Ifc6afa4a5fe35977135428f6d0e9f7c164767720
Reviewed-on: https://go-review.googlesource.com/23312Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/h2_bundle.go

@@ -3111,6 +3111,7 @@ type http2stream struct {
 	sentReset        bool // only true once detached from streams map
 	gotReset         bool // only true once detacted from streams map
 	gotTrailerHeader bool // HEADER frame for trailers was seen
+	wroteHeaders     bool // whether we wrote headers (not status 100)
 	reqBuf           []byte
 
 	trailer    Header // accumulated trailers
@@ -3474,7 +3475,21 @@ func (sc *http2serverConn) writeFrameFromHandler(wm http2frameWriteMsg) error {
 // If you're not on the serve goroutine, use writeFrameFromHandler instead.
 func (sc *http2serverConn) writeFrame(wm http2frameWriteMsg) {
 	sc.serveG.check()
-	sc.writeSched.add(wm)
+
+	var ignoreWrite bool
+
+	switch wm.write.(type) {
+	case *http2writeResHeaders:
+		wm.stream.wroteHeaders = true
+	case http2write100ContinueHeadersFrame:
+		if wm.stream.wroteHeaders {
+			ignoreWrite = true
+		}
+	}
+
+	if !ignoreWrite {
+		sc.writeSched.add(wm)
+	}
 	sc.scheduleFrameWrite()
 }