Column assignment doesn't work when comma is omitted

When assigning values to an entire column, if the comma is omitted, the assignment is made to the row instead.

For example:

``` R
> dat <- data.frame(diag(100))
> x <- BigDataFrame(data=dat)
> x[1] <- 3
> x[1:5, 1:5]
  X1 X2 X3 X4 X5
1  3  3  3  3  3
2  0  1  0  0  0
3  0  0  1  0  0
4  0  0  0  1  0
5  0  0  0  0  1

```
