org-element: Print lower case affiliated keywords

* lisp/org-element.el (org-element--interpret-affiliated-keywords):
  Print lower case keywords.

lisp/org-element.el

@@ -4581,7 +4581,7 @@ If there is no affiliated keyword, return the empty string."
 	    (let (dual)
 	      (when (member key org-element-dual-keywords)
 		(setq dual (cdr value) value (car value)))
-	      (concat "#+" key
+	      (concat "#+" (downcase key)
 		      (and dual
 			   (format "[%s]" (org-element-interpret-data dual)))
 		      ": "