Fix CallOrderError: Y called before Y [fixes issue #96]

If I do something like this
x.should_receive('y').and_return(a).once().ordered()
x.should_receive('y').and_return(b).once().ordered()

then I expect the mocked method to be called twice with the same arguments
(none in this case), returning a different value the second time (b).
Before this fix, it instead resulted in a CallOrderError exception.