[R-Forge #5377] data.table(data.frame()) creates an empty V1 column, should be 0 columns

Submitted by: shubh bansal; Assigned to: Arun ; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5377&group_id=240&atid=975)

``` S
data.frame()
data frame with 0 columns and 0 rows

data.table(data.frame())
Empty data.table (0 rows) of 1 col: V1
```

Ideally the `data.table` should have 0 columns
