add Write method to RawPacket struct

[vany-egorov]

if you want to copy incoming RTP packet as-is. For example
when you need to redirect RTP packet from WebRTC to plain RTP:

pkt := rtp.DataPacket{}

pkt.Write(buf.Bytes())
// or io.Copy(&pkt, buf)

session.WriteData(&pkt)

[vany-egorov]

Hi!

Any comments?