Fixes for dcast.data.table

+ Workaround for segfault in dcast.data.table due to existing bug #5191.
+ Error in dcast.data.table if input DT has duplicated names
+ Error if data to cast is empty
+ Added more tests to dcast.data.table
+ Minor fixes in ?dcast.data.table

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1226 edb9625f-4e0d-4859-8d74-9fd3b1da38cb