convert 2dim DataArray to multi-column DataFrame

Previously:
```
julia> @data([1 NA; 3 4; 5 6])
3x2 DataArray{Int64,2}:
 1   NA
 3  4  
 5  6  

julia> convert(DataFrame, ans)
6x1 DataFrame
|-------|----|
| Row # | x1 |
| 1     | 1  |
| 2     | NA |
| 3     | 3  |
| 4     | 4  |
| 5     | 5  |
| 6     | 6  |
```

I think it's expected that these should be converted to multiple columns:
```
julia> convert(DataFrame, ans)
2x3 DataFrame
|-------|----|----|----|
| Row # | x1 | x2 | x3 |
| 1     | 1  | 3  | 5  |
| 2     | NA | 4  | 6  |
```