fix sort for data.table

[setempler]

This works:

sort(data.frame(a=3:1))

But this doesn't:

sort(data.table(a=3:1))
# Error in `[.data.table`(x, , by, drop = F) : 
#  j (the 2nd argument inside [...]) is a single symbol but column name 'by' is not found. Perhaps you intended DT[,..by] or DT[,by,with=FALSE]. This difference to data.frame is deliberate and explained in FAQ 1.1.