p2p: fix marshaling of NAT in TOML

This fixes an issue where a nat.Interface unmarshaled from the
TOML config file could not be re-marshaled to TOML correctly.

p2p/config.go

@@ -18,6 +18,7 @@ package p2p
 
 import (
 	"crypto/ecdsa"
+	"encoding"
 	"fmt"
 
 	"github.com/ethereum/go-ethereum/common/mclock"
@@ -135,6 +136,13 @@ type configNAT struct {
 	nat.Interface
 }
 
+func (w *configNAT) MarshalText() ([]byte, error) {
+	if tm, ok := w.Interface.(encoding.TextMarshaler); ok {
+		return tm.MarshalText()
+	}
+	return nil, fmt.Errorf("NAT specification %#v cannot be marshaled", w.Interface)
+}
+
 func (w *configNAT) UnmarshalText(input []byte) error {
 	n, err := nat.Parse(string(input))
 	if err != nil {