Use http_response_code() to check HTTP status code, when available.

Resolves WebSharks/quick-cache#175
Minor tweak. https://github.com/WebSharks/quick-cache/issues/175
Merge branch 'b-quick-cache-175' of github.com:WebSharks/Quick-Cache into b-quick-cache-175
Merge branch 'b-quick-cache-175' into 140520-RC