BUG: #3125 preserve .nii.gz extension on scene save

Use completeSuffix per suggestion from jcfr.

Note this code may still change as I work on fixes for #2326.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22246 3bd1e089-480b-0410-8dfb-8563597acbee