ox-latex: Allow markup objects inside %D spec

* lisp/ox-latex.el (org-latex--format-spec): Pass date through
`org-export-data' just as other backends do.

TINYCHANGE

lisp/ox-latex.el

@@ -1888,7 +1888,7 @@ INFO is a plist used as a communication channel."
       (?c . ,(plist-get info :creator))
       (?l . ,language)
       (?L . ,(capitalize language))
-      (?D . ,(org-export-get-date info)))))
+      (?D . ,(org-export-data (org-export-get-date info) info)))))
 
 (defun org-latex--insert-compiler (info)
   "Insert LaTeX_compiler info into the document.