Delegate shouldn't be able to reassign task

Steps to reproduce:

1. Start a parallel workflow and assign a task to jdoe.
2. As jdoe, delegate the task to jsmith.
   -> Both jdoe and jsmith can process the task. jdoe is still shown as responsible for the task.
3. As jsmith, go on the Process view of the task in Home > Workflow.
4. Reassign the task to jlennon.
   -> Jlennon is now shown as the responsible for the task.
   -> jdoe doesn't have the task on his workflow dashboard anymore.

I'm not sure a delegate should be allowed to reassign the task and thus remove the responsible for the task. jsmith can even reassign the task to himself.