reading icmpv6 packet payload with ApplicationLayer.Payload() includes last 4 bytes of icmpv6 header

Hello! I am receiving packets that can be icmpv4 or icmpv6, so I would like to read their payloads like this:

```golang
appLayer := packet.ApplicationLayer()
icmpPayload := appLayer.Payload()
````

When I receive an ICMPv4 packet, the payload doesn't include the icmp message body, or the final 4 bytes of the icmp header.
When I receive an ICMPv6 packet, the payload includes the icmp message body.

So for an identical payload received from some device, eg of 58 bytes, I would receive 58 bytes from an ICMPv4 packet decoded as above, and 62 from an ICMPv6 packet decoded as above.

Please let me know if this is expected!