Allow :var header args to be assigned to cons cells

* lisp/ob-core.el (org-babel-disassemble-tables,
org-babel-gnuplot-process-vars): Use `proper-list-p' check instead of
`listp'.
(org-babel-disassemble-tables): Check list length before trying to
remove hline from a table assignment.
* testing/lisp/test-ob.el (test-ob/cons-cell-as-variable): Add the
test.

lisp/ob-core.el

@@ -1712,9 +1712,10 @@ of the vars, cnames and rnames."
     (list
      (mapcar
       (lambda (var)
-        (when (listp (cdr var))
+        (when (proper-list-p (cdr var))
           (when (and (not (equal colnames "no"))
-                     (or colnames (and (eq (nth 1 (cdr var)) 'hline)
+                     (or colnames (and (length> (cdr var) 1)
+                                       (eq (nth 1 (cdr var)) 'hline)
                                        (not (member 'hline (cddr (cdr var)))))))
             (let ((both (org-babel-get-colnames (cdr var))))
               (setq cnames (cons (cons (car var) (cdr both))

lisp/ob-gnuplot.el

@@ -88,7 +88,7 @@ code."
        (cons
 	(car pair) ;; variable name
 	(let* ((val (cdr pair)) ;; variable value
-	       (lp  (listp val)))
+	       (lp  (proper-list-p val)))
 	  (if lp
 	      (org-babel-gnuplot-table-to-data
 	       (let* ((first  (car val))

testing/lisp/test-ob.el

@@ -178,6 +178,22 @@ should still return the link."
 			    (point-at-bol)
 			    (point-at-eol))))))
 
+(ert-deftest test-ob/cons-cell-as-variable ()
+  "Test that cons cell can be assigned as variable."
+  (org-test-with-temp-text "
+
+#+name: cons
+#+begin_src emacs-lisp
+  (cons 1 2)
+#+end_src
+
+#+begin_src emacs-lisp :var x=cons
+  x
+#+end_src"
+
+    (org-babel-next-src-block 2)
+    (should (equal (cons 1 2) (org-babel-execute-src-block)))))
+
 (ert-deftest test-ob/multi-line-header-arguments ()
   "Test that multi-line header arguments and can be read."
   (org-test-with-temp-text-in-file "