webdav: fix incorrect modtime after setting a file's modtime

SetModTime kept the caller's full precision modtime in memory while
the PROPPATCH sets it on the server with second precision, so until
the object was re-read the modtime did not match what a fresh listing
would report. This broke wrappers which compare exact modtimes, such
as the hasher backend's fingerprint and the VFS cache. Truncate the
modtime to second precision to match what the server stores.

backend/webdav/webdav.go

@@ -1499,8 +1499,11 @@ func (o *Object) SetModTime(ctx context.Context, modTime time.Time) error {
 		}
 		// FIXME check if response is valid
 		if len(result.Responses) == 1 && result.Responses[0].Props.StatusOK() {
-			// update cached modtime
-			o.modTime = modTime
+			// update cached modtime - the PROPPATCH sets it with
+			// second precision so truncate here too to keep the
+			// in-memory modtime identical to the one a fresh
+			// listing returns
+			o.modTime = modTime.Truncate(time.Second)
 			return nil
 		}
 		// got an error, but it's possible it actually worked, so double-check