When a relation field defines multiple proxy listbox ids, don't use to the first one, use the default.
The previous behaviour to use the first one is kept if there is only one proxy listbox id.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@29121 20353a03-c40f-0410-a6d1-a30d3c3de9de