Test X-Cache behaviour when Origin already sets it

We want to confirm that any X-Cache header set by Origin is
preserved, with the Edge cache status appended to the end of the
header.
Add 'origin does not set X-Cache' test case

If Origin does not set an X-Cache header - for example if we
filter it out in the origin caches - we want to confirm that the
Edge will still set the header as we expect.
Merge pull request #39 from alphagov/x_cache_header

X-Cache header behaviour tests