Use correct parameter list to _shutdown()

[JasonDeArte]

This update fixes a param call typo, to a member function 2 lines below it.

Previous to this change, the "cancel_msg" param would get a boolean instead of a prefered string.

[tschoonj]

Can someone merge this in please? It is a trivial fix for an obvious bug.

[tschoonj]

See also #117 and #105