The endpoint POST /runtime/tasks/{taskId} ignores the scope of the variables for the "complete" action

The [documentation of the endpoint](https://www.flowable.org/docs/userguide/index.html#_task_actions) indicates that this is the expected behaviour but it's a bit misleading that you are able to provide a scope that is going to be ignored.

Another benefit of changing the endpoint to make it work with the provided scopes is that the REST API user will be able to update variables and complete the task in a single request, that is, much more easy and convenient transactionality.