webdav: return multistatus with 403 on PROPPATCH

This fixes a "The file cannot be accessed by the system" error when uploading
some files with Windows Explorer.

server.go

@@ -189,8 +189,51 @@ func (b *backend) propFindFile(propfind *internal.PropFind, fi *FileInfo) (*inte
 }
 
 func (b *backend) PropPatch(r *http.Request, update *internal.PropertyUpdate) (*internal.Response, error) {
-	// TODO: return a failed Response instead
+	fi, err := b.FileSystem.Stat(r.Context(), r.URL.Path)
-	return nil, internal.HTTPErrorf(http.StatusForbidden, "webdav: PROPPATCH is unsupported")
+	if err != nil {
+		return nil, err
+	}
+
+	resp := &internal.Response{Hrefs: []internal.Href{internal.Href{Path: fi.Path}}}
+
+	for _, set := range update.Set {
+		for _, raw := range set.Prop.Raw {
+			xmlName, ok := raw.XMLName()
+			if !ok {
+				continue
+			}
+
+			emptyVal := internal.NewRawXMLElement(xmlName, nil, nil)
+
+			if err := resp.EncodeProp(http.StatusForbidden, emptyVal); err != nil {
+				return nil, err
+			}
+		}
+
+	}
+
+	for _, remove := range update.Remove {
+		for _, raw := range remove.Prop.Raw {
+			xmlName, ok := raw.XMLName()
+			if !ok {
+				continue
+			}
+
+			emptyVal := internal.NewRawXMLElement(xmlName, nil, nil)
+
+			if err := resp.EncodeProp(http.StatusForbidden, emptyVal); err != nil {
+				return nil, err
+			}
+		}
+
+	}
+
+	if len(resp.PropStats) == 0 {
+		return nil, internal.HTTPErrorf(http.StatusBadRequest,
+			"webdav: request missing properties to update")
+	}
+
+	return resp, nil
 }
 
 func (b *backend) Put(w http.ResponseWriter, r *http.Request) error {