Make every struct have a header header and not inherit the header

pdu/pdu.go

@@ -22,7 +22,7 @@ type (
 	}
 
 	GENERIC_NACK struct {
-		PDU_HEADER
+		header PDU_HEADER
 	}
 )
 
@@ -63,4 +63,4 @@ func (p *PDU_HEADER) Decode(data []uint8) error {
 }
 func (p *PDU_HEADER) Size() uint32 {
 	return 16
-}
+}