Fix regression on RVKCs that was introduced ca3c3a7b56d7c82441e1071a658f3e0537a97d48, along with some tests to ensure it doesn't happen again.

git-svn-id: http://svn6.assembla.com/svn/pyfits/branches/3.2-stable@2619 ed100bfc-0583-0410-97f2-c26b58777a21