packet interface now requires both un/marshal

It only required unmarshal before. Needed to add stub method to extended
packet so all packets would implement the interface.

This allows packet interface to be used for both incoming and outgoing
packets.

request-packet.go

@@ -8,6 +8,7 @@ import (
 
 // all incoming packets
 type packet interface {
+	encoding.BinaryMarshaler
 	encoding.BinaryUnmarshaler
 	id() uint32
 }
@@ -47,6 +48,11 @@ func (p sshFxpReaddirPacket) getHandle() string  { return p.Handle }
 // this has a handle, but is only used for close
 func (p sshFxpClosePacket) getHandle() string { return p.Handle }
 
+// for packet struct uniformity, so they all implement packet interface
+func (s *sshFxpExtendedPacket) MarshalBinary() ([]byte, error) {
+	return []byte{}, nil
+}
+
 // take raw incoming packet data and build packet objects
 func makePacket(p rxPacket) (packet, error) {
 	var pkt packet