Allow type override in := when i is just order() #2925

DT = data.table(a = 1:10)
DT[order(-a), a := paste0(a)]

This errors saying we should be explicit about type overrides, but:

DT[ , a := paste0(a)]

This code works fine (and is of course identical, but the same problem arises when the column update is order-dependent). Presumably this is because when i is non-empty, := suspects there could be a type collision if assigning a new type to a subset of rows.

But order queries in i are incapable of subsetting, so I would expect the behavior to be identical.