webdav: fix interop with davrods server

The davrods server returns URLS with a double / in and the // confuses
rclone into thinking these files are in a directory called "".

The fix removes leading /s from the directory listing names.

See: https://forum.rclone.org/t/upload-to-webdav-does-not-check-if-files-already-exist/35756/

backend/webdav/webdav.go

@@ -712,6 +712,7 @@ func (f *Fs) listAll(ctx context.Context, dir string, directoriesOnly bool, file
 			continue
 		}
 		subPath := u.Path[len(baseURL.Path):]
+		subPath = strings.TrimPrefix(subPath, "/") // ignore leading / here for davrods
 		if f.opt.Enc != encoder.EncodeZero {
 			subPath = f.opt.Enc.ToStandardPath(subPath)
 		}