When splitting keywords make sure the continuation line does not start with //...

When splitting keywords make sure the continuation line does not start with // since Isis Pvl considers that a comment.  Fixes #1230.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5181 41f8697f-d340-4b68-9986-7bafba869bb8