net/http: update bundled http2 for Server WriteTimeout change

Updates http2 to x/net/http2 git rev 8fd7f25 for:

    http2: clear WriteTimeout in Server
    https://golang.org/cl/34724

And un-skip the new test. (The new test is a slow test, anyway, so
won't affect builders or all.bash, but I verified it now passes.)

Updates #18437

Change-Id: Ia91ae702edfd23747a9d6b61da284a5a957bfed3
Reviewed-on: https://go-review.googlesource.com/34729
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Kale B <kale@lemnisys.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/serve_test.go

@@ -537,7 +537,6 @@ func TestServerTimeouts(t *testing.T) {
 
 // Test that the HTTP/2 server handles Server.WriteTimeout (Issue 18437)
 func TestHTTP2WriteDeadlineExtendedOnNewRequest(t *testing.T) {
-	t.Skip("disabled until Issue 18437 is fixed")
 	if testing.Short() {
 		t.Skip("skipping in short mode")
 	}