Closes bug #5415. .BY gets names attribute set properly.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1258 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Closes #5527. Related to #5423. More tests added. News Updated.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1259 edb9625f-4e0d-4859-8d74-9fd3b1da38cb