fix #1744 GenericForeignKey has no value_from_object()
fix #1744 GenericForeignKey has no value_from_object()

lino-framework/lino,khchine5/lino,khchine5/lino,lsaffre/lino,lino-framework/lino,lsaffre/lino,lino-framework/lino,khchine5/lino,lsaffre/lino,lino-framework/lino,lsaffre/lino,lino-framework/lino,khchine5/lino,khchine5/lino,lsaffre/lino