Fix AttributeError on timeout with async

oc = zerorpc.Client(connect_to="tcp://127.0.0.1:8003")
oc.test('whats up', async=True).wait()

If there is a timeout:
An exception is raised: "AttributeError: 'str' object has no attribute 'name'"

Thanks jvangael for the bug report.

This patch also add a test case.