Generate PROPFIND request body

Instead of a hardcoded string, generate it with encoding/xml.

client.go

@@ -1,9 +1,9 @@
 package webdav
 
 import (
+	"encoding/xml"
 	"fmt"
 	"net/http"
-	"strings"
 
 	"github.com/emersion/go-webdav/internal"
 )
@@ -21,21 +21,15 @@ func NewClient(c *http.Client, endpoint string) (*Client, error) {
 }
 
 func (c *Client) FindCurrentUserPrincipal() (string, error) {
-	r := strings.NewReader(`<?xml version="1.0" encoding="utf-8"?>
-<D:propfind xmlns:D="DAV:">
-  <D:prop>
-    <D:current-user-principal/>
-  </D:prop>
-</D:propfind>
-`)
+	name := xml.Name{"DAV:", "current-user-principal"}
+	propfind := internal.NewPropPropfind(name)
 
-	req, err := c.c.NewRequest("PROPFIND", "", r)
+	req, err := c.c.NewXMLRequest("PROPFIND", "", propfind)
 	if err != nil {
 		return "", err
 	}
 
 	req.Header.Add("Depth", "0")
-	req.Header.Add("Content-Type", "text/xml; charset=\"utf-8\"")
 
 	resps, err := c.c.DoMultiStatus(req)
 	if err != nil {

internal/client.go

@@ -1,6 +1,7 @@
 package internal
 
 import (
+	"bytes"
 	"encoding/xml"
 	"fmt"
 	"io"
@@ -36,6 +37,27 @@ func (c *Client) NewRequest(method string, p string, body io.Reader) (*http.Requ
 	return http.NewRequest(method, u.String(), body)
 }
 
+func (c *Client) NewXMLRequest(method string, p string, v interface{}) (*http.Request, error) {
+	var buf bytes.Buffer
+	buf.WriteString(xml.Header)
+	enc := xml.NewEncoder(&buf)
+	if err := enc.Encode(v); err != nil {
+		return nil, err
+	}
+	if err := enc.Flush(); err != nil {
+		return nil, err
+	}
+
+	req, err := c.NewRequest(method, p, &buf)
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Add("Content-Type", "text/xml; charset=\"utf-8\"")
+
+	return req, nil
+}
+
 func (c *Client) Do(req *http.Request) (*http.Response, error) {
 	// TODO: remove this quirk
 	req.SetBasicAuth("simon", "")

internal/elements.go

@@ -27,3 +27,19 @@ type Propstat struct {
 	Status  string      `xml:"DAV: status"`
 	// TODO: error?, responsedescription?
 }
+
+// https://tools.ietf.org/html/rfc4918#section-14.20
+type Propfind struct {
+	XMLName xml.Name     `xml:"DAV: propfind"`
+	Prop    *RawXMLValue `xml:"DAV: prop,omitempty"`
+	// TODO: propname | (allprop, include?)
+}
+
+func NewPropPropfind(names ...xml.Name) *Propfind {
+	children := make([]RawXMLValue, len(names))
+	for i, name := range names {
+		children[i] = *NewRawXMLElement(name, nil, nil)
+	}
+	prop := NewRawXMLElement(xml.Name{"DAV:", "prop"}, nil, children)
+	return &Propfind{Prop: prop}
+}

internal/xml.go

@@ -13,6 +13,11 @@ type RawXMLValue struct {
 	children []RawXMLValue
 }
 
+// NewRawXMLElement creates a new RawXMLValue for an element.
+func NewRawXMLElement(name xml.Name, attr []xml.Attr, children []RawXMLValue) *RawXMLValue {
+	return &RawXMLValue{tok: xml.StartElement{name, attr}, children: children}
+}
+
 // UnmarshalXML implements xml.Unmarshaler.
 func (val *RawXMLValue) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
 	val.tok = start