Handle calling `side_effect` and `return_value` with spies

[ollipa]

Right now, nothing happens if side_effect or return_value is called when spying. Should a a warning or an exception be raised if those methods are called? Should a custom logic be implemented to match the return value of the spy so these methods could actually be useful with spies?

[ollipa]

Now it's not possible to assert return values in spies. These methods could be used for that:

# This would assert that "add_tea" method returns "loose green tea" and test would fail if it doesn't
>>> mocker(teapot).spy("add_tea").return_value("loose green tea")
>>> teapot.add_tea("green tea", loose=True)

# This would assert that the "temperature" method first returns value 98 and then raises and exception on second call
>>> mocker(teapot).spy("temperature").side_effect(98, RuntimeError("Too hot"))
>>> teapot.temperature()
>>> teapot.temperature()

Unittest doesn't support this directly but we could save the return values of the passthrough object when spying and then validate the return values during teardown.