Do not marshal 'empty' error elements

iq.go

@@ -77,16 +77,26 @@ func (x *Err) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
 }
 
 func (x Err) MarshalXML(e *xml.Encoder, start xml.StartElement) (err error) {
+	if x.Code == 0 {
+		return nil
+	}
+
+	// Encode start element and attributes
+	start.Name = xml.Name{Local: "error"}
+
 	code := xml.Attr{
 		Name:  xml.Name{Local: "code"},
 		Value: strconv.Itoa(x.Code),
 	}
+	start.Attr = append(start.Attr, code)
+
+	if len(x.Type) > 0 {
 		typ := xml.Attr{
 			Name:  xml.Name{Local: "type"},
 			Value: x.Type,
 		}
-	start.Name = xml.Name{Local: "error"}
-	start.Attr = append(start.Attr, code, typ)
+		start.Attr = append(start.Attr, typ)
+	}
 	err = e.EncodeToken(start)
 
 	// SubTags

iq_test.go

@@ -2,6 +2,7 @@ package xmpp // import "fluux.io/xmpp"
 
 import (
 	"encoding/xml"
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -51,6 +52,10 @@ func TestGenerateIq(t *testing.T) {
 		t.Errorf("cannot marshal xml structure")
 	}
 
+	if strings.Contains(string(data), "<error ") {
+		t.Error("empty error should not be serialized")
+	}
+
 	parsedIQ := IQ{}
 	if err = xml.Unmarshal(data, &parsedIQ); err != nil {
 		t.Errorf("Unmarshal(%s) returned error", data)