Allow setting default clocking task to current clocking task
* lisp/org-clock.el (org-clock-in): Set default clocking task when already clocking the task
The default clocking task can now be set to the current clocking task.
Previously this just threw an error stating the clock continues in the
current clocking task. The double prefix now forces setting the
default clocking task instead of forcing the user to clock out and
back in again just to set the default clocking task to the current
clocking task.