http2: handle pings in Transport

Change-Id: I50f7084bca2e5f7adb4cf278a32251f5bb239dbe
Reviewed-on: https://go-review.googlesource.com/16731
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
diff --git a/http2/server.go b/http2/server.go
index 7405cda..86069c2 100644
--- a/http2/server.go
+++ b/http2/server.go
@@ -1076,7 +1076,7 @@
 
 func (sc *serverConn) processPing(f *PingFrame) error {
 	sc.serveG.check()
-	if f.Flags.Has(FlagSettingsAck) {
+	if f.IsAck() {
 		// 6.7 PING: " An endpoint MUST NOT respond to PING frames
 		// containing this flag."
 		return nil