fixed key extraction problems with non-copyable classes reported at http://lists.boost.org/Archives/boost/2006/04/103065.php

[SVN r33732]

libs/multi_index

@@ -1 +1 @@
-Subproject commit 9a782276a48ef0afe823dce0fef23b43f5299687
+Subproject commit 5ecebcbc784e83a5019e4d7d0cad17815250ccb8