Implement MarshalXML for StringMap type

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/handler/response.go

@@ -86,3 +86,31 @@ type LocationResponse struct {
 	XMLName  xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ LocationConstraint" json:"-"`
 	Location string   `xml:",chardata"`
 }
+
+// MarshalXML - StringMap marshals into XML.
+func (s StringMap) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+
+	tokens := []xml.Token{start}
+
+	for key, value := range s {
+		t := xml.StartElement{}
+		t.Name = xml.Name{
+			Space: "",
+			Local: key,
+		}
+		tokens = append(tokens, t, xml.CharData(value), xml.EndElement{Name: t.Name})
+	}
+
+	tokens = append(tokens, xml.EndElement{
+		Name: start.Name,
+	})
+
+	for _, t := range tokens {
+		if err := e.EncodeToken(t); err != nil {
+			return err
+		}
+	}
+
+	// flush to ensure tokens are written
+	return e.Flush()
+}