Fix make_optional for rvalues

doc/html/index.html

@@ -26,7 +26,8 @@
 <span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
 </h3></div></div>
 <div><p class="copyright">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal</p></div>
-<div><p class="copyright">Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski</p></div>
+<div><p class="copyright">Copyright &#169; 2014-2017 Andrzej
+      Krzemie&#324;ski</p></div>
 <div><div class="legalnotice">
 <a name="optional.legal"></a><p>
         Distributed under the Boost Software License, Version 1.0. (See accompanying
@@ -145,7 +146,7 @@
 </div>
 </div>
 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
-<td align="left"><p><small>Last revised: March 22, 2017 at 21:55:47 GMT</small></p></td>
+<td align="left"><p><small>Last revised: May 17, 2017 at 23:01:39 GMT</small></p></td>
 <td align="right"><div class="copyright-footer"></div></td>
 </tr></table>
 <hr>