Prevent invoked expectations from being modified after they are invoked.

I paired with @mrgilman on this. I'm happy to carry the pull request over the line based on feedback.
> Also, "Whilst" is such a british-ism (I can't think if any time I've ever heard an American use that word). Mind using "while" instead so that this doesn't sound old-fashioned to American users?

@mrgilman said the exact same thing :P I guess that's just my extreme britishness coming through. I'll definitely change it.