Presolver.detach_model() should cause subsequent access to model to fail

[JoelPasvolsky]

The detach_model() docs say "Subsequent attempts to access the model will raise a RuntimeError" but that is not happening:

pre = Presolver(cqm)
pre.load_default_presolvers()
pre.detach_model()
cqm2 = pre.copy_model()
>>> type(cqm2)
dimod.constrained.constrained.ConstrainedQuadraticModel
>>> cqm2.is_equal(dimod.ConstrainedQuadraticModel())
True

The post-detach cqm2 = pre.copy_model() command gives no error and happily generates an empty CQM.