proper way to modify or replace an application-layer payload

Huge fan of gopacket, thanks for all the great work! I'm working on a project that leverages it heavily. We deal with pcaps and have a need to obscure some packet payloads read from a file before writing the modified packet out to a new file. The goal is to leave the rest of the packet untouched, but replace payload bytes with a useless character ("*" for example).

I've used the approach described [in Issue 765](https://github.com/google/gopacket/issues/765) with some success. It seems to work fine for TCP, even with other app layer protocols supported by gopacket like HTTP. However when the following block encounters a UDP DNS packet it panics with `panic: interface conversion: gopacket.ApplicationLayer is *layers.DNS, not *gopacket.Payload`.

```
if appLayer := packet.ApplicationLayer(); appLayer != nil {
payloadLen := len(appLayer.LayerContents())
var newPayload = make([]byte, payloadLen)
for i := range newPayload {
newPayload[i] = blank
}
*packet.ApplicationLayer().(*gopacket.Payload) = newPayload
}
```

I've also tried doing this at the Transport layer by replacing the layer's payload directly. But this feels wrong knowing that the payload has already been decoded as DNS.

```
payloadLen := len(packet.TransportLayer().LayerPayload())
var newPayload = make([]byte, payloadLen)
for i := range newPayload {
newPayload[i] = blank
}

if udp := packet.TransportLayer().(*layers.UDP); udp != nil {
udp.Payload = newPayload
} else if tcp := packet.TransportLayer().(*layers.TCP); tcp != nil {
udp.Payload = newPayload
}
```

Not surprisingly gopacket.SerializePacket() throws an error `layer DecodeFailure is not serializable` when I then try to serialize the packet with the modified transport payload. I'm clearly missing a step or going about this wrong. Perhaps there is a way to reinitialize the modified packet telling gopacket not to treat it as DNS anymore?

I've also considered assembling a new packet from the layers of the original, but swapping the payload for a new `gopacket.Payload`. This could work but means I'm creating another copy of the packet. I'd rather modify the original in place if possible. But what is the correct way?