org-element-set-contents: Do alter the anonymous parents

* lisp/org-element.el (org-element-set-contents): Do alter anonymous
elements (el1 el2 ...).  Such elements are used, for example, when
parsing keyword values during export, like :title.
* testing/lisp/test-org-element.el (test-org-element/set-contents):
Add test.

The patch fixed bug during export when exporting a subtree with
option stat:nil.  The :title during subtree export is taken from the
heading title and parsed.  However, the parsed value is stored outside
the parse tree, in :title property of the INFO channel.  The parsed
value does get filtered through `org-export--prune-tree', but before
this commit, `org-element-set-contents' did not actually alter the
out-of-AST-tree parent lists of elements.

Reported-by: Leo Butler <Leo.Butler@umanitoba.ca>
Link: https://orgmode.org/list/87mt4w8epo.fsf@t14.reltub.ca

lisp/org-element.el

@@ -548,7 +548,14 @@ Return modified element."
   "Set ELEMENT's contents to CONTENTS.
 Return ELEMENT."
   (cond ((null element) contents)
-	((not (symbolp (car element))) contents)
+	((not (symbolp (car element)))
+         (if (not (listp element))
+             ;; Non-element.
+             contents
+           ;; Anonymous element (el1 el2 ...)
+           (setcar element (car contents))
+           (setcdr element (cdr contents))
+           element))
 	((cdr element) (setcdr (cdr element) contents) element)
 	(t (nconc element contents))))
 

testing/lisp/test-org-element.el

@@ -125,7 +125,13 @@ Some other text
    (org-test-with-temp-text "* Headline\n *a*"
      (let ((tree (org-element-parse-buffer)))
        (org-element-set-contents (org-element-map tree 'bold 'identity nil t))
-       (org-element-contents (org-element-map tree 'bold 'identity nil t))))))
+       (org-element-contents (org-element-map tree 'bold 'identity nil t)))))
+  ;; Set contents of anonymous elements.
+  (should
+   (equal '#1=((b (:parent #1#)))
+          (let ((element '#1=((a (:parent #1#)) (b (:parent #1#)))))
+            (org-element-set-contents element `(b (:parent ,element)))
+            element))))
 
 (ert-deftest test-org-element/secondary-p ()
   "Test `org-element-secondary-p' specifications."