Fixes #5379 - dcast with factor cols on LHS along with drop=FALSE

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1172 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
+ Faster := and set() to a new column by not filling with NA first where we know entire vector will be written
+ set() no longer duplicates atomic NAMED==0 plonked columns

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1173 edb9625f-4e0d-4859-8d74-9fd3b1da38cb