Fix bug in with-location-override macro, update unit test

When I refactored the `(with-location-override ...)` macro, I didn't realize it could take any combination of overrides. I was only grabbing the first key/value pair from the `override` map. Whoops! :confounded:

The existing unit test didn't catch this case, so I've updated it to fail on the existing code. The new implementation of the override macro now passes the improved test, and it should also override any new values you might want to add without modification.

Once you've had a chance to review this, if it looks clean, would you mind deploying a new RC build to clojars? I can then pull the new version into my app and determine if this fixes the bug I was seeing.

Also, I'm curious to hear your thoughts (if any) about the top-level let bindings we were discussing earlier.

Thanks for your time!