Fix make_optional for rvalues

doc/html/index.html

@@ -146,7 +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: November 04, 2017 at 18:02:17 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>