webdav: allow empty host in move

interpret empty host as current

Change-Id: I70be8aa33c0e501df22a8ad17b7b430620d45da6
GitHub-Last-Rev: 28224d33dfbf265d7cb22b890107e40a28818a45
GitHub-Pull-Request: golang/net#44
Reviewed-on: https://go-review.googlesource.com/c/net/+/181698
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

webdav/webdav.go

@@ -323,7 +323,7 @@ func (h *Handler) handleCopyMove(w http.ResponseWriter, r *http.Request) (status
 	if err != nil {
 		return http.StatusBadRequest, errInvalidDestination
 	}
-	if u.Host != r.Host {
+	if u.Host != "" && u.Host != r.Host {
 		return http.StatusBadGateway, errInvalidDestination
 	}