[O] custom IDs not exported

[Daniel Clemente]

  Commit 438536f6157794101ce0957e39cad6bf70580751 („Change underscores to 
hyphens in section labels“) broke the export of CUSTOM_IDs.

  Take for instance this file:

-----------------------
#+TITLE: Cosas por hacer

* Programas
  :PROPERTIES:
  :CUSTOM_ID: programas
  :END:

** emacs
   :PROPERTIES:
   :CUSTOM_ID: emacs
   :END:
Editor de textos

------------------------


  When exporting (C-c C-e H) you get this table of contents:

------------------------
<div id="text-table-of-contents">
<ul>
<li><a href="#programas">1 Programas </a>
<ul>
<li><a href="#sec-1_1">1.1 emacs </a></li>
</ul>
</li>
</ul>
</div>

-------------------------

  The first one is correct because it is first-level. All non-first-level get 
#sec-NUMBERS instead of the custom ID.

  Not only the table of contents; the header's ID is also wrong:
<h3 id="sec-1_1"><a name="sec-1-1" id="sec-1-1"></a><span 
class="section-number-3">1.1</span> emacs </h3>


  Tested with current org-mode. 


  Greetings,
Daniel