babel: variables can now accept null literal values

* lisp/babel/ob-exp.el (org-babel-exp-results): now checking to see if
  return value of `org-babel-ref-literal' is equal to the null
  indicator flag -- meaning it's now possible to accept a value of
  null

* lisp/babel/ob-ref.el (org-babel-ref-parse):  now checking to see if
  return value of `org-babel-ref-literal' is equal to the null
  indicator flag -- meaning it's now possible to accept a value of
  null

  (org-babel-ref-literal): now returning a null indicator flag when
  the value is *not* a literal value, meaning it is possible to pass
  in a literal value of null

lisp/babel/ob-exp.el

@@ -245,7 +245,8 @@ results into the buffer."
 	    (if (and org-current-export-file
 		     (eq (car pair) :var)
 		     (string-match org-babel-ref-split-regexp (cdr pair))
-		     (null (org-babel-ref-literal (match-string 2 (cdr pair)))))
+		     (equal :ob-must-be-reference
+			    (org-babel-ref-literal (match-string 2 (cdr pair)))))
 		`(:var . ,(concat (match-string 1 (cdr pair))
 				  "=" org-current-export-file
 				  ":" (match-string 2 (cdr pair))))

lisp/babel/ob-ref.el

@@ -83,8 +83,10 @@ emacs-lisp representation of the value of the variable."
       (let ((var (match-string 1 assignment))
             (ref (match-string 2 assignment)))
         (cons (intern var)
-              (or (org-babel-ref-literal ref)
-                  (org-babel-ref-resolve-reference ref params))))))
+	      ((lambda (val)
+		 (if (equal :ob-must-be-reference val)
+		     (org-babel-ref-resolve-reference ref params)
+		   val)) (org-babel-ref-literal ref))))))
 
 (defun org-babel-ref-literal (ref)
   "Determine if the right side of a header argument variable
@@ -95,7 +97,8 @@ return nil."
   (let ((out (org-babel-read ref)))
     (if (equal out ref)
         (if (string-match "^\".+\"$" ref)
-            (read ref))
+            (read ref)
+	  :ob-must-be-reference)
       out)))
 
 (defvar org-babel-library-of-babel)