Fix savePolygonFile* ASCII file support

[VictorLamoine]

Without the fix the following code outputs 2 binary files using VTK trunk:

  pcl::PolygonMesh mesh;
  pcl::io::loadPolygonFile("/home/victor/input.ply", mesh);
  pcl::io::savePolygonFile("/home/victor/output_1.ply", mesh, true);
  pcl::io::savePolygonFile("/home/victor/output_2.ply", mesh, false);

I think the VTK guys switched again from ASCII to binary default format somewhere between VTK 6.1 and VTK trunk, not sure when exactly. We already had that discussion here and here.

🐌 🐢