Fixes #873

Note: This solution does incur a small performance penalty in that each
callback first checks if the view is still open. Only one other unit test was
affected by the change, because it used a callback which executed after close
of view, and if I understood the other unit test correctly, it remained solved.
A couple other options would be rewriting several things in Backbone (unbind
algorithm, event trigger method, etc), or writing a more specific override
which only kicks in if the callback method has some special property.