setorder sets 'key' to NULL by default now. Not sure if it makes sense for setorder to check and retain key in certain special cases, but that's to revisit later.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1213 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Another corner case in 'setorder'. setorder(NULL) now returns 'x', not 'invisible(x)'.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1214 edb9625f-4e0d-4859-8d74-9fd3b1da38cb