In some cases, we may want to use lists (like in parallel list fields), so it's
not good to always value to string. If a conversion (with '\n' addition and
'\r' replacement) is needed, this should be done in the render_od* method

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31525 20353a03-c40f-0410-a6d1-a30d3c3de9de