make expected due_date value format explicit

app/views/shared/issuable/_sidebar.html.haml

@@ -97,7 +97,7 @@
                   remove due date
           - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
             .selectbox.hide-collapsed
-              = f.hidden_field :due_date, value: issuable.due_date
+              = f.hidden_field :due_date, value: issuable.due_date.try(:strftime, 'yy-mm-dd')
               .dropdown
                 %button.dropdown-menu-toggle.js-due-date-select{ type: 'button', data: { toggle: 'dropdown', field_name: "#{issuable.to_ability_name}[due_date]", ability_name: issuable.to_ability_name, issue_update: issuable_json_path(issuable) } }
                   %span.dropdown-toggle-text Due date