spy_on() can now be called on an unbound function on a class (such as

ClassName.my_function), which is useful when the calling code doesn't

have a particular instance it's working on, but needs to spy on an

instance that will be created in the future.
This works just like spying on any other function.

Spied on an unbounded function in a unit test, was able to override the

handler for an instance created in the future.