fix error response in case of wrong expire time format

weed/s3api/s3api_object_handlers.go

@@ -63,7 +63,7 @@ func (s3a *S3ApiServer) PutObjectHandler(w http.ResponseWriter, r *http.Request)
 
 	if r.Header.Get("Expires") != "" {
 		if _, err = time.Parse(http.TimeFormat, r.Header.Get("Expires")); err != nil {
-			s3err.WriteErrorResponse(w, r, s3err.ErrInvalidDigest)
+			s3err.WriteErrorResponse(w, r, s3err.ErrMalformedExpires)
 			return
 		}
 	}