webdav: fix Open Range requests to fix 4shared mount

Before this change the webdav backend didn't truncate Range requests
to the size of the object. Most webdav providers are OK with this (it
is RFC compliant), but it causes 4shared to return 500 internal error.

Because Range requests are used in mounting, this meant that mounting
didn't work for 4shared.

This change truncates the Range request to the size of the object.

See: https://forum.rclone.org/t/cant-copy-use-files-on-webdav-mount-4shared-that-have-foreign-characters/21334/

backend/webdav/webdav.go

@@ -1128,6 +1128,7 @@ func (o *Object) Storable() bool {
 // Open an object for read
 func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.ReadCloser, err error) {
 	var resp *http.Response
+	fs.FixRangeOption(options, o.size)
 	opts := rest.Opts{
 		Method:  "GET",
 		Path:    o.filePath(),