Trapped DT[,sum(.SD),by=] with error suggesting lapply(.SD,sum) or base::sum(.SD), #5380.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1174 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Removed bug report on unreleased features

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1175 edb9625f-4e0d-4859-8d74-9fd3b1da38cb