convert object_id to str. In this way, line uid will not be unicode, and can be concatenate with others ascii words (like it is in Products/Formulator/Widget.py l. 856)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@29539 20353a03-c40f-0410-a6d1-a30d3c3de9de