Fixed a problem with our Column class that caused failures in Isis' automated...

Fixed a problem with our Column class that caused failures in Isis' automated tests. This bug could have caused cubediff to throw errors unexpectedly when using the outputdiffs option. Fixes #1125.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4718 41f8697f-d340-4b68-9986-7bafba869bb8