Squashed commit of the following:

- Minor fix to anyDuplicated
- Closes bug #5437. `by=.` cols now don't lose attributes when used in `j`. Better tests added to catch further regressions. News updated.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1256 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Updated NEWS alone.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1257 edb9625f-4e0d-4859-8d74-9fd3b1da38cb