project.task: use an onchange to update date_start when changing user_id.
Indeed date_start is automatically updated in the create / write. Without
the onchange, this may lead to errors related to date_start being greater
then date_end.
project.issue: code in create and write now takes into account date_open
values given to the method and avoid erasing them.
In issue however the onchange is not added. Indeed the date_open and
date_closed fields are not visible in the view. They are automatically
computed and used to compute statistics.
Thibault Delavallée
8fa7318178