Support spying on unbound functions on classes.
Review Request #6132 — Created July 22, 2014 and submitted
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. Reviewed at https://reviews.reviewboard.org/r/5910/