Auto indexing and integer key with numeric RHS

``` R
require(data.table) # 1.9.5
dt = data.table(x=1:5, y=6:10)
options(datatable.auto.index=TRUE)
dt[x == 2.5] # wrong
#    x y
#1: 2 7
options(datatable.auto.index=FALSE)
dt[x == 2.5]
# Empty data.table (0 rows) of 2 cols: x,y
```

Fix on the way..
