Fix mayavi tests after upgrading to vtk 5.10.1 (old pipeline)

1. This fixes the following 2 tests: test_poly_data_reader.py and test_image_data_reader.py as the HeaderSize attribute of vtkImageReader2 was not a LongType in the traits wrapper.

2. ***[WIP]***: Fixing SphereWidget enable/disable which fails test_mlab_integration.py and premature EOF errors for vtkPLOT3DReader.