BUG #2379. Passing flags correctly to internal writers.

Fix for bUG #2379 added new ivars. Ensuring that vtkXMLDataSetWriter and
vtkXMLPDataSetWriter pass these to the internal writers correctly.

Change-Id: I916b7c3e2e703b309ef1b02dc25e16409c5a3b29
Merge topic 'bug_2379' into master

444257e5 BUG #2379. Passing flags correctly to internal writers.