Add Test for X-Cache 'follow on' behaviour?

[mikepea]

Raising this issue as a discussion point. When developing [#39] we pondered whether to add a subsequent test to check that the [HIT, MISS] Header would turn to [HIT, HIT] on a subsequent request for the now cached object.

This is more complicated than the base checks, since it requires a pause before the check to ensure the CDN cache has been distributed locally -- and we're not sure it's worth the extra effort/test-time-increase required.