Add test for X-Cache-Hits behaviour

Slightly different to what we originally expected, the Edge appends its
own hit count to the X-Cache-Hits header.
Use a const to explain magic value
Merge branch 'master' into x_cache_hits
Add mock VCL to emulate X-Cache-Hits behaviour
Merge pull request #10 from alphagov/x_cache_hits

Add test for X-Cache-Hits behaviour