speedup for ad hoc dogroups, avoiding memcpy for single items

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1138 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Raised a too-deep setAttrib in dogroups

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1139 edb9625f-4e0d-4859-8d74-9fd3b1da38cb