[FIX] mail: chatter: follower subtype: check checkbox on unfollow cancel

When unfollowing the last followed subtype, a confirm dialog is shown to the
user to confirm that it will unfollow the document. If he clicked on 'Cancel',
the unchecked subtype wasn't correctly re-checked.

addons/mail/static/src/js/chatter.js

@@ -413,7 +413,7 @@ var Followers = form_common.AbstractField.extend({
         // If no more subtype followed, unsubscribe the follower
         if (!checklist.length) {
             if (!this.do_unfollow(ids)) {
-                $(event.target).attr("checked", "checked");
+                $(event.target).prop("checked", true);
             } else {
                 self.$('.o_subtypes_list ul').empty();
             }