net/http: update bundled http2

Update http2 to x/net git rev db473f6b23.

(And un-skip TestWriteHeader0_h2 added in CL 80077, now fixed.)

Includes:

   http2: remove afterReqBodyWriteError wrapper
   https://golang.org/cl/75252

   http2: fix transport data race on reused *http.Request objects
   https://golang.org/cl/75530

   http2: require either ECDSA or RSA ciphersuite
   https://golang.org/cl/30721

   http2: don't log about timeouts reading client preface on new connections
   https://golang.org/cl/79498

   http2: don't crash in Transport on server's DATA following bogus HEADERS
   https://golang.org/cl/80056

   http2: panic on invalid WriteHeader status code
   https://golang.org/cl/80076

   http2: fix race on ClientConn.maxFrameSize
   https://golang.org/cl/79238

   http2: don't autodetect Content-Type when the response has an empty body
   https://golang.org/cl/80135

Fixes golang/go#18776
Updates golang/go#20784
Fixes golang/go#21316
Fixes golang/go#22721
Fixes golang/go#22880

Change-Id: Ie86e24e0ee2582a5a82afe5de3c7c801528be069
Reviewed-on: https://go-review.googlesource.com/80078
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>

src/net/http/clientserver_test.go

@@ -1391,9 +1391,6 @@ func TestBadResponseAfterReadingBody(t *testing.T) {
 func TestWriteHeader0_h1(t *testing.T) { testWriteHeader0(t, h1Mode) }
 func TestWriteHeader0_h2(t *testing.T) { testWriteHeader0(t, h2Mode) }
 func testWriteHeader0(t *testing.T, h2 bool) {
-	if h2 {
-		t.Skip("skipping until CL 80076 is vendored into std")
-	}
 	defer afterTest(t)
 	gotpanic := make(chan bool, 1)
 	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {