Added 2 tests for rbindlist'ing factors containing NA.
Added 2 tests for rbindlist'ing factors containing NA.

Rdatatable/data.table,vlulla/data.table,vlulla/data.table,vlulla/data.table,jangorecki/data.table,jangorecki/data.table,Rdatatable/data.table,Rdatatable/data.table,jangorecki/data.table,vlulla/data.table,jangorecki/data.table,Rdatatable/data.table