add method to get highest in order ACKed packet number to new ACK frame

frames/ack_frame_new_test.go

@@ -62,6 +62,16 @@ var _ = Describe("AckFrame", func() {
 		})
 	})
 
+	Context("highest in order packet number", func() {
+		It("gets the hightest in order packet number for a simple ACK", func() {
+			frame := &AckFrameNew{
+				LargestObserved: 0x1337,
+			}
+			Expect(frame.GetHighestInOrderPacketNumber()).To(Equal(protocol.PacketNumber(0x1337)))
+		})
+
+	})
+
 	Context("self-consistency checks", func() {
 		var b *bytes.Buffer
 		BeforeEach(func() {