Fixes #59

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2688 ed100bfc-0583-0410-97f2-c26b58777a21
Fixes #58 with a special case to prevent duplicate insertion of commentary keywords

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2689 ed100bfc-0583-0410-97f2-c26b58777a21