The ComputeScalars option adds the array from which the contour was
computed to the output. vtkContourFilter used to contour by the active
scalar array in the point data, but now contours by the point data
array set with vtkAlgorithm::SetInputArrayToProcess(). However, some
aspects of copying the data are still controlled by which array is
designated as active. The result is that when the active scalar array
is not the same as the one named with SetInputArrayToProcess(), the
active point data array will not be added to the output if the
ComputeScalars option is off, and the contouring array will be added
incorrectly.

This problem manifested only when the input data set type was
vtkUnstructuredGrid or vtkPolyData. Other data types are handled by
helper classes where arrays are handled correctly. This patch fixes
the problem by setting the active scalar array in the input data to
match the input array to process. An existing test for unstructured
grid input was modified to test the expected behavior and a new test
for vtkPolyData was added.