Error when unchecking the milestone in an issue

To clear a milestone, I tried to uncheck the milestone instead of selecting "No milestone", this action leads to a stuck UI and an error in logs.

Steps to reproduce:

1. Choose an issue with a milestone
2. Open the milestone selector
3. Uncheck the selected milestone
4. The loading indicator keeps spinning forever while there is an error in logs

Logs:

app_1     | Started PUT "/tas/bo/issues/139.json" for 127.0.0.1 at 2016-06-15 08:18:27 +0000
app_1     | Processing by Projects::IssuesController#update as JSON
app_1     |   Parameters: {"namespace_id"=>"tas", "project_id"=>"bo", "id"=>"139"}
app_1     | Completed 400 Bad Request in 38ms (ActiveRecord: 3.7ms)
app_1     | 
app_1     | ActionController::ParameterMissing (param is missing or the value is empty: issue):
app_1     |   app/controllers/projects/issues_controller.rb:206:in `issue_params'
app_1     |   app/controllers/projects/issues_controller.rb:95:in `update'
app_1     |   lib/gitlab/middleware/go.rb:16:in `call'