Make sure issue assignee is properly reset.

Previously, when the assignee was reset via the sidebar or bulk edit, `assignee_id` was set to `-1` rather than `null`, which caused the two issues shown below:

![Screen_Shot_2015-03-24_at_16.52.13](https://gitlab.com/gitlab-org/gitlab-ce/uploads/3c937795c45031c3c72c124ced866598/Screen_Shot_2015-03-24_at_16.52.13.png)

- A "(deleted)" participant
- An empty selectbox in the sidebar, instead of "Select assignee"

See merge request !443