all: more testing and attr refactor

errorcode.go

@@ -1,6 +1,9 @@
 package stun
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 // ErrorCodeAttribute represents ERROR-CODE attribute.
 type ErrorCodeAttribute struct {
@@ -8,6 +11,10 @@ type ErrorCodeAttribute struct {
 	Reason []byte
 }
 
+func (c ErrorCodeAttribute) String() string {
+	return fmt.Sprintf("%d: %s", c.Code, c.Reason)
+}
+
 // constants for ERROR-CODE encoding.
 const (
 	errorCodeReasonStart = 4