new-matrix bug with dataset implementation fixed

Bug:
```Clojure
user=> (new-matrix :dataset 3 4)
#clojure.core.matrix.impl.dataset.DataSet{:column-names [nil nil nil nil], :columns [[0.0 0.0 0.0] [0.0 0.0 0.0] [0.0 0.0 0.0] [0.0 0.0 0.0]]}
```

After applying PR:
```Clojure
user=> (new-matrix :dataset 3 4)
#clojure.core.matrix.impl.dataset.DataSet{:column-names [:0 :1 :2 :3], :columns [[0.0 0.0 0.0] [0.0 0.0 0.0] [0.0 0.0 0.0] [0.0 0.0 0.0]]}
```