forder speedup for character

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1121 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
fixed reverse sort of 2nd column in dradix

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1122 edb9625f-4e0d-4859-8d74-9fd3b1da38cb