Improve last todo-mode fix

* lisp/calendar/todo-mode.el (todo-read-category): Use
set-keymap-parent instead of copy-keymap, and default (as
previously) to the global binding (for rationale, see
http://lists.gnu.org/archive/html/emacs-devel/2016-06/msg00217.html).

lisp/calendar/todo-mode.el

@@ -5751,8 +5751,9 @@ against categories in that file; otherwise complete against all
 categories from `todo-category-completions-files'."
   ;; Allow SPC to insert spaces, for adding new category names.
   (let ((minibuffer-local-completion-map
-         (copy-keymap minibuffer-local-completion-map)))
-    (define-key minibuffer-local-completion-map " " 'self-insert-command)
+         (let ((map (make-sparse-keymap)))
+           (set-keymap-parent map minibuffer-local-completion-map)
+           (define-key map " " nil))))
     (let* ((add (eq match-type 'add))
 	   (archive (eq match-type 'archive))
 	   (file0 (when (and file (> (length todo-files) 1))