Merge pull request #6 from fidian/master

Fixing caching header to be maximum date allowed on XP
I had the logic backwards.

If the status is truthy (ie. 200, 201, 400, 500, etc), then we want everything possible.  If the status is falsy (undefined, 0, empty string), then something is severely wrong and we can't assume anything is really there or anything is really correct.