BUG: #3125 filename not changed if it is still default from load

The file load mechanism assigns a default node name based on
the filename.  If the node no longer has that default name, update
the filename to match what the user provided.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22259 3bd1e089-480b-0410-8dfb-8563597acbee