Incorrect SparseColumnMatrix.numSlices() causes IndexException in toString()

SparseColumnMatrix overrides the numSlices() method incorrectly: it returns numCols() instead of numRows().
As a result, AbstractMaxtrix.toString() for wide matrices throws an exception.
For example, this code:

Matrix matrix = new SparseColumnMatrix(1, 2);
matrix.toString();

causes

org.apache.mahout.math.IndexException: Index 1 is outside allowable range of [0,1)
at org.apache.mahout.math.MatrixVectorView.<init>(MatrixVectorView.java:42)
at org.apache.mahout.math.AbstractMatrix.viewRow(AbstractMatrix.java:290)
at org.apache.mahout.math.AbstractMatrix$1.computeNext(AbstractMatrix.java:68)
at org.apache.mahout.math.AbstractMatrix$1.computeNext(AbstractMatrix.java:59)
at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
at org.apache.mahout.math.AbstractMatrix.toString(AbstractMatrix.java:787)