Fixed "internal value of TRUE has been modified" with R 3.1 when all groups are size 1 and there's a logical column.  Wasn't filed on tracker, just datatable-help (1 May 2014).

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1280 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-svn-local'