This package is required, largely for testing, so that Guzzle requests
can be mocked instead of making live calls to the API.

This is a simplistic implementation of the endpoint. It's not complete,
yet, but it's a start. Its implementation is required to implement the
one call API endpoint.

I didn't look deeply enough at Location previously to realise that City
already extended it. Given that, this change effectively reverts the
previous refactor of Location to put the changes where they should be,
on City.

As the method's private, it can't be tested directly. However, that's
not to say that it cannot be tested. So this test does that to ensure
that the URL it returns is always correct.