TWCC header ext interceptor cause FEC packet corruption

[3DRX]

Your environment.

https://github.com/3DRX/fec-test

What did you do?

Register these 3 interceptors in this order

	fecInterceptor, err := flexfec.NewFecInterceptor()
	if err != nil {
		panic(err)
	}
	twccInterceptor, err := twcc.NewHeaderExtensionInterceptor()
	if err != nil {
		panic(err)
	}
	nackResponder, err := nack.NewResponderInterceptor()
	if err != nil {
		panic(err)
	}
	i.Add(twccInterceptor)
	i.Add(fecInterceptor)
	i.Add(nackResponder)

What did you expect?

This should be the correct order to place these three interceptors, but the video gets corrupted.

What happened?

Found this during debugging #318 , probably because of twcc interceptor's bug when setting header for fec packets.