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.
Well, you are British :).

I think we should aim to make all text produced by RSpec non-jarring to native English speakers from anywhere in the world.  Of course, we may not reach that aim (particularly since we don't have core team members for each major dialect) but it's a good thing to aim for :).

If there are any Americanisms I (or any other contributors) use that sound jarring to your ears, please speak up :).

Spelling is another matter, and I don't think we'll be able to completely solve that one :(...