org-table: Fix `org-table-copy-down' with timestamps

* lisp/org-table.el (org-table-copy-down): Allow matching inactive
  timestamps.

Reported-by: Alan Schmitt <alan.schmitt@polytechnique.org>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-09/msg00304.html>

lisp/org-table.el

@@ -1184,7 +1184,7 @@ to a number.  In the case of a timestamp, increment by days."
 	  (setq txt (calc-eval (concat txt "+" (number-to-string inc)))))
       (insert txt)
       (org-move-to-column col)
-      (if (and org-table-copy-increment (org-at-timestamp-p))
+      (if (and org-table-copy-increment (org-at-timestamp-p 'lax))
 	  (org-timestamp-up-day inc)
 	(org-table-maybe-recalculate-line))
       (org-table-align)