[7.0] Task followers deleted after edition.

[Ceferino Cuevas]

1) Steps to reproduce the issue you have observed
Create a Project and add followers.
Create a task without a project and assign it to a user.
Edit the new task and assign it to the new project.
Notice that the user that has assigned the task is no longer a follower of it after asign the task to the new project.

Can find a demonstrative video here:
http://www.youtube.com/watch?v=mo2wYDL0Ijc

2) The result you observed
The user who was assigned to the task is deleted from the follower list after assign the task to the project.

3) The result you expected
A followers merge between the user who owns the task and the followers of the project.

4) The platform your are using
Ubuntu 12.04

5) The OpenERP version
http://7-0-12681.runbot.openerp.com/?db=7-0-12681-all

[Moisés López - http://www.vauxoo.com (moylop260)]

We test it
lp:~openerp-dev/openobject-addons/7.0-opw-593459-pn

The fix is done!