In LSpacePreserver#unbind, use arity to decide whether to pass a reason

We caught this with em-http-request in particular, which uses the
unbind reason for connection-related errors (i.e. ECONNREFUSED).

It's not the most elegant implementation, but I don't know any other
way to find the method that "super" will call.
++test
Merge branch 'ebroder/unbind-reason'
v0.8