as.data.table() returns a 1 column matrix when the matrix columns are factors

[mgahan]

Here is some sample data
iris <- data.table(iris)

This code produces a one column vector when Species is a factor.
as.data.table(combn(unique(iris$Species),2))

V1
1: setosa
2: versicolor
3: setosa
4: virginica
5: versicolor
6: virginica

While this code produces a matrix.
as.data.table(combn(unique(as.character(iris$Species)),2))

V1 V2 V3
1: setosa setosa versicolor
2: versicolor virginica virginica

Issue found through SO question here:
http://stackoverflow.com/questions/26244685/r-count-every-possible-pair-of-values-in-a-column-grouped-by-multiple-columns/26246588?noredirect=1#comment41177317_26246588

[arunsrinivasan]

@mgahan 👍 thank you.