Add more complete tests for the OPTIONS NodeJS vs Varnish bug.

OPTIONS requests were failing when NodeJS was proxying them to Varnish
(since NodeJS mangled OPTIONS requests without bodies and Varnish
no-likey). This was patched in the gatekeeper app, but this is a better
full integration test to ensure all HTTP methods work with the full
stack in place.

See:
https://github.com/NREL/api-umbrella-gatekeeper/commit/e8745f2721a8e75ad687fc1459efb045550bf664