minor fixes to `setDT`
minor fixes to `setDT`

    `setDT` lost ExtPtr ref. is now fixed.
    Class attributes to data.frame are now preserved.

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