Automatic indexing and ! operator

Thanks to Matthieu for the email.

``` R
require(data.table)
DT <- data.table(v1 = c(1,NA,2), v2 = c(1,NA,2))

DT[!(v1==1)]
#    v1 v2
#1:  2  2
DT[!v1==1] ## different result due to auto indexing
#    v1 v2
#1: NA NA
#2:  2  2
options(datatable.auto.index=FALSE)
DT[!v1==1] ## turning off restores default functionality
#    v1 v2
#1:  2  2
```
