BUG: GDCM Series does not write z-spacing.

When a GDCMImageIO is used with an ImageSeriesWriter, the z-spacing is not
written to the output image.  The ITK_Spacing metadata passed was not being
added to the output.  m_NumberOfDimensions is only 2 in this case.  If there
is a z-spacing that is set, it should be passed.

Change-Id: Ia8744c7d391e2e80e06b7843224f44a5398a55e5
Merge branch 'gdcm-spacing-release3' into release

Change-Id: I921407e67211743e4057c95bb4145b688e75dc38