Fix poly data reader intermittent failing test

Refer PR#220 and #231.

The poly data reader test saving and restoring visualization test fails, but only intermittently, mostly on Windows, reported by Appveyor.

The error trace:

```
======================================================================
FAIL: Test if saving a visualization and restoring it works.
----------------------------------------------------------------------
Traceback (most recent call last):
File "C:\projects\mayavi\mayavi\tests\test_poly_data_reader.py", line 182, in test_save_and_restore
self.check_saving(self.e, self.scene, self.bounds)
File "C:\projects\mayavi\mayavi\tests\data_reader_test_base.py", line 75, in check_saving
self.check(scene,bounds,error)
File "C:\projects\mayavi\mayavi\tests\data_reader_test_base.py", line 59, in check
self.assertEqual(numpy.allclose(got, bounds, atol=error), True, msg)
AssertionError: Got: (0.0, 1.0, 0.0, 1.0, 0.0, 2.8244394598286436e+29); expected: (0.0, 1.0, 0.0, 1.0, 0.0, 1.6)
-------------------- >> begin captured logging << --------------------
mayavi.core.registry: DEBUG: Engine [] named NullEngine1 registered
--------------------- >> end captured logging << ---------------------
```