Accept keys for org-export-as-html-to-buffer' and `org-export-region-as-html'

* org-exp.el (org-export-visible): Accept keys for
`org-export-as-html-to-buffer' and `org-export-region-as-html'.

H and R were not in the list of allowed keys but bound to the
respective function later.

Missing keys reported by Samuel Wales <samologist@gmail.com>.

lisp/org-exp.el

@@ -2682,7 +2682,7 @@ command."
 	   (message "Export visible: [a]SCII  [h]tml  [b]rowse HTML [H/R]buffer with HTML  [D]ocBook  [l]atex  [p]df  [d]view pdf  [L]atex buffer  [x]OXO  [ ]keep buffer")
 	   (read-char-exclusive))
 	 current-prefix-arg))
-  (if (not (member type '(?a ?n ?u ?\C-a ?b ?\C-b ?h ?D ?x ?\ ?l ?p ?d ?L)))
+  (if (not (member type '(?a ?n ?u ?\C-a ?b ?\C-b ?h ?D ?x ?\ ?l ?p ?d ?L ?H ?R)))
       (error "Invalid export key"))
   (let* ((binding (cdr (assoc type
 			      '(