org-duration: `org-duration-to-minutes' accepts plain numbers

* lisp/org-duration.el (org-duration-to-minutes): Accept plain numbers
  for backward compatibility with `org-hh:mm-string-to-minutes'.

* testing/lisp/test-org-duration.el (test-org-duration/to-minutes):
  Add test.

Reported-by: Dale <dale@codefu.org>
<http://permalink.gmane.org/gmane.emacs.orgmode/112290>

lisp/org-duration.el

@@ -279,6 +279,7 @@ As a special case, a bare number represents minutes.
 Return value as a float.  Raise an error if duration format is
 not recognized."
   (cond
+   ((numberp duration) (float duration))
    ((string-match-p org-duration--h:mm-re duration)
     (pcase-let ((`(,hours ,minutes ,seconds)
 		 (mapcar #'string-to-number (split-string duration ":"))))

testing/lisp/test-org-duration.el

@@ -34,6 +34,7 @@
   ;; Special case: a bare number is treated as minutes.
   (should (= (org-duration-to-minutes "2") 2))
   (should (= (org-duration-to-minutes "2.5") 2.5))
+  (should (= (org-duration-to-minutes 1) 1))
   ;; Support custom units.
   (should (= 4
 	     (let ((org-duration-units '(("longmin" . 2)))