Allow overriding a stub func with return values

Now if a method stub function is set, and then later return values are set,
the fakes will return the new return values instead of continuing to use
the method stub

It seems like this behavior would be a little bit easier to understand since the most recent 'fake configuration' is what will be returned, instead of an internal priority/ordering that user's probably shouldn't need to be aware of. It could be considered a bit of a breaking change though.