Example data.

```
require(data.table)
dt <- data.table(rnorm(10), rnorm(10) < 0.5)

```

Compute correlation between numeric and logical variables gives error.

```
cor(dt)
#Error in cor(dt) : 'x' must be numeric

```

But error goes away when converting to a data frame.

```
cor(data.frame(dt))
#           V1         V2
#V1  1.0000000 -0.1631356
#V2 -0.1631356  1.0000000

```

Is this intended behaviour for `data.table`?